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Studies of incidents in medicine and other fields attribute most bad out-
comes to a category of human performance labeled
human error.
example, surveys of anesthetic incidents in the operating room have attrib-
uted between 70% and 82% of the incidents surveyed to the human element
(Chopra, Bovill, Spierdijk, & Koornneef, 1992; Cooper, Newbower, Long,
& McPeek, 1978). Similar surveys in aviation have attributed more than
70% of incidents to crew error (Boeing Product Safety Organization, 1993).
In general, incident surveys in a variety of industries attribute similar per-
centages of critical events to human error (for example, see Hollnagel, 1993,
Table 1). The result is the perception, in both professional and lay communi-
ties, of a "human error problem" in medicine, aviation, nuclear power
generation, and similar domains. To cope with this perceived unreliability of
people, it is conventional to try to reduce or regiment the human's role in a
risky system by enforcing standard practices and work rules and by using
automation to shift activity away from people.
Generally, the "human" referred to when an incident is ascribed to
human error is some individual or team of practitioners who work at what
Reason calls the "sharp end" of the system (Reason, 1990; Fig. 13.1).
Practitioners at the sharp end actually interact with the hazardous process in
their roles as pilots, physicians, spacecraft controllers, or power plant opera-
tors. In medicine, these practitioners are anesthesiologists, surgeons, nurses,
and some technicians who are physically and temporally close to the patient.
Those at the "blunt end" of the system, to continue Reason's analogy, affect
safety through their effect on the constraints and resources acting on the
practitioners at the sharp end. The blunt end includes the managers, system
architects, designers, and suppliers of technology. In medicine, the blunt end
includes government regulators, hospital administrators, nursing managers,
and insurance companies. In order to understand the sources of expertise
and error at the sharp end, one must also examine this larger system to see
how resources and constraints at the blunt end shape the behavior of sharp-
end practitioners (Reason, 1990). This chapter examines issues surrounding
human performance at the sharp end, including those described as errors
and those considered expert.
Most people use the term
human error
to delineate one category of
potential causes for unsatisfactory activities or outcomes. Human error as a
cause of bad outcomes is used in engineering approaches to the reliability of
complex systems (probabilistic risk assessment) and is widely used in inci-
dent-reporting systems in a variety of industries. For these investigators,
human error is a specific variety of human performance that is, in retrospect,
so clearly and significantly substandard and flawed that there is no doubt
that the practitioner should have viewed it as substandard
at the time the act
was committed.
The judgment that an outcome was due to human error is an
attribution that (a) the human performance immediately preceding the
incident was unambiguously flawed, and (b) the human performance led
directly to the outcome.
But the term "human error" is controversial (e.g., Hollnagel, 1993).
Attribution of error is
about human performance. These judg-
ments are rarely applied except when an accident or series of events have
occurred that could have or nearly did end with a bad outcome. Thus, these
judgments are made ex post facto, with the benefit of
about the
outcome or near miss. These factors make it difficult to attribute specific
incidents and outcomes to "human error" in a consistent way. Fundamental
questions arise. When precisely does an act or omission constitute an "er-
ror"? How does labeling some act as a human error advance our under-
standing of why and how complex systems fail? How should we respond to
incidents and errors to improve the performance of complex systems? These
are not academic or theoretical questions. They are close to the heart of
tremendous bureaucratic, professional, and legal conflicts and tied directly
to issues of safety and responsibility. Much hinges on being able to deter-
mine how complex systems have failed and on the human contribution to
such outcome failures. Even more depends on judgments about what means
will prove effective for increasing system reliability, improving human per-
formance, and reducing or eliminating human errors.
Studies in a variety of fields show that the label "human error" is prejudi-
cial and unspecific. It retards rather than advances our understanding of how
complex systems fail and the role of human practitioners in both successful
and unsuccessful system operations. The investigation of the cognition and
behavior of individuals and groups of people, not the attribution of error in
itself, points to useful changes for reducing the potential for disaster in large,
complex systems. Labeling actions and assessments as "errors" identifies a
symptom, not a cause; the symptom should call forth a more in-depth
investigation of how a system of people, organizations, and technologies
functions and malfunctions (Hollnagel,1993; Rasmussen, Duncan, & Leplat,
1987; Reason, 1990; Woods, Johannesen, Cook, & Sarter, 1994).
Recent research into the evolution of system failures finds that the story
of "human error" is markedly complex (Hollnagel,1993; Rasmussen et al.,
1987; Reason, 1990; Woods et al., 1994). For example:
The context in which incidents evolve plays a major role in human
performance at the sharp end.
Technology can shape human performance, creating the potential for
new forms of error and failure.
The human performance in question usually involves a set of interact-
ing people.
People at the blunt end create dilemmas and shape trade-offs among
competing goals for those at the sharp end.
The attribution of error after the fact is a process of social judgment
rather than a scientific conclusion.
The goal of this chapter is to provide an introduction to the complexity of
system failures and the term
human error.It
may seem simpler merely to
attribute poor outcomes to human error and stop there. If one looks beyond
the label, the swirl of factors and issues seems very complex. But it is in the
examination of these deeper issues that one can learn how to improve the
performance of large, complex systems.
We begin with an introduction to the complexity of error through several
exemplar incidents taken from anesthesiology. Each of these incidents may
be considered by some to contain one or more human errors. Careful
examination of the incidents, however, reveals a more complicated story
about human performance. The incidents provide a way to introduce some
of the research results about the factors that affect human performance in
complex settings such as medicine. Because the incidents are drawn from
anesthesiology, most of the discussion is about human performance in the
conduct of anesthesia, but the conclusions apply to other medical specialties
and even to other domains.
The second part of the chapter deals more generally with the failures of
large, complex systems and the sorts of problems those who would analyze
human performance in such systems must encounter. It is significant that the
results from studies in medicine and other domains such as aviation and
nuclear power plant operation are parallel and strongly reinforcing. The
processes of cognition are not fundamentally different between practitio-
ners in these domains, and the problems that practitioners are forced to deal
with are quite similar.
We should not be surprised that the underlying
features of breakdowns in these large, complex systems are quite similar.
Grappling with the complexity of human error and system failure has
strong implications for the many proposals to improve safety by restructur-
ing the training of people, introducing new rules and regulations, and adding
technology. The third part of the chapter explores the consequences of these
ideas for attempts to eliminate "human error" as a cause of large, complex
system failures.
What factors affect the performance of practitioners in complex settings like
medicine? Figure 13.1 provides a schematic overview. For practitioners at
the sharp end of the system, there are three classes of cognitive factors that
govern how people form intentions to act:
Knowledge factors-factors related to the knowledge that can be
drawn on when solving problems in context.
Attentional dynamics-factors that govern the control of attention
and the management of mental workload as situations evolve and
change over time.
3. Strategic factors-the trade-offs between goals that conflict, espe-
cially when the practitioners must act under uncertainty, risk, and the
pressure of limited resources (e.g., time pressure; opportunity costs).
These three classes are depicted as interlocking rings at the sharp end of
the operational system because these functions overlap. Effective system
operation depends on their smooth integration within a single practitioner
and across teams of practitioners. The figure does not show a single indi-
vidual because these categories are not assigned to individuals in a one-to-
one fashion. Rather, they are distributed and coordinated across multiple
people and across the artifacts they use.
These factors govern the expression of error and expertise together with
two other classes of factors. First are the demands placed on practitioners by
characteristics of the incidents and problems that occur. These demands
vary in type and complexity. One incident may present itself as a textbook
version of a problem for which a well-practiced plan is available and appro-
priate. A different incident may appear embedded in a complicated back-
ground of interacting factors, creating a substantial cognitive challenge for
FIG. 13.1.
The sharp and blunt ends of a large complex system. The interplay of
problem demands and the resources of practitioners at the sharp end govern the
expression of expertise and error. The resources available to meet problem demands
are shaped and constrained in large part by the organizational context at the blunt end
of the system (from Woods et al., 1994, reprinted by permission).
practitioners. The problem demands shape the cognitive activities of those
confronting the incident at the sharp end.
The second broad class of factors arises from the blunt end of the system
and includes the resources and constraints under which the practitioners
function. Recent work on human error has recognized the importance of the
organizational context
in system failures (Reason, 1990, chap. 7). This con-
text influences both the physical and cognitive resources available to practi-
tioners as they deal with the system. For example, the knowledge available
during system operations is, in part, the result of the organization's invest-
ments in training and practice. Similarly, the organizational context influ-
ences how easy it is to bring more specialized knowledge to bear as an
incident evolves and escalates. Finally, organizational context tends to set
up or sharpen the strategic dilemmas practitioners face. Thus, organiza-
tional (blunt end) factors provide the context in which the practitioners'
Knowledge Factors: Incident #1 -Myocardial Infarction
An elderly patient presented with a painful, pulseless, blue arm indicating a
blood clot (embolus) in one of the major arteries that threatened loss of that
mb. Emergency surgery to perform removal of the clot (embolectomy) was
clearly indicated. The patient had a complex medical and surgical history with
high blood pressure, diabetes requiring regular insulin treatment, a prior heart
attack, and previous coronary artery bypass surgery. The patient also had
evidence of recently worsening congestive heart failure, that is, shortness of
breath, dyspnea on exertion and leg swelling (pedal edema). Electrocardio-
gram changes included inverted T waves. Chest X-ray suggested pulmonary
edema. The arterial blood gas showed markedly low oxygen in the arterial
blood (P.02 of 56 on unknown F0
The blood glucose was high (800). The
patient received furosemide (a diuretic) and 12 units of insulin in the emer-
gency room. The patient was taken to the operating room for removal of the
clot under local anesthesia with sedation provided by the anesthetist. In the
operating room the patient's blood pressure was high, 210/120; a nitroglycerin
drip was started and increased in an effort to reduce the blood pressure. The
arterial oxygen saturation
was 88% on nasal cannula and did not
mprove with a rebreathing mask, but rose to the high 90s when the anesthesia
machine circuit was used to supply 100% oxygen by mask. The patient did not
complain of chest pain but did complain of abdominal pain and received
morphine. Urine output was high in the operating room. The blood pressure
continued about 200/100. Nifedipine was given sublingually and the pressure
fell over 10 minutes to 90 systolic. The nitroglycerin infusion rate was de-
creased and the pressure rose to 140. The embolectomy was successful. Post-
operative cardiac enzyme studies showed a peak about 12 hours after the
surgical procedure, indicating that the patient had suffered a myocardial
infarction (heart attack) sometime in the period including the time in the
emergency room and the operating room. The patient survived.'
This incident raises a host of issues regarding the nature of knowledge and
its use during the evolution of the incident. Knowledge factors include those
related to the knowledge available for solving problems. Especially impor-
tant are those factors that conditionalize knowledge toward its use, that is,
those that "call knowledge to mind." In Incident #1, it is clear that the
participant was employing a great deal of knowledge. In fact, the description
of just a few of the relevant aspects of knowledge important to the incident
occupies several pages.
There is evidence that the participant was missing or misunderstanding
mportant, but less obvious features of the case. It seems (and seemed to
peer experts who evaluated the incident at the time; cf., Cook, Woods, &
McDonald, 1991) that the practitioner misunderstood the nature of the
patient's intravascular volume, believing the volume was high rather than
low. This increased volume is often present in patients with the signs of
congestive heart failure. In this case, however, other factors (including the
high blood glucose and the prior treatment with a diuretic) were present that
indicated that the patient should be treated differently. In retrospect, other
practitioners argued that the patient probably should have received more
intravenous fluid to replenish the low intravascular volume. They also felt
that the patient should have been monitored invasively to allow precise
determination of when enough fluid had been given (e.g., a catheter that
goes through the heart and into the pulmonary artery).
It is also apparent that many of the practitioner's actions were appropri-
This incident comes from Cook, Woods and McDonald, 1991 which examined a corpus
cases in anesthesiology and the associated human performance issues.
ate in the context of the case as it evolved. For example, the level of oxygen
in the blood was low and the anesthetist pursued several different means of
increasing the blood oxygen level, including the use of oxygen by mask.
Similarly, the blood pressure was high, and this too was treated, first with
nitroglycerin (which may lower the blood pressure but also can protect the
heart by increasing its blood flow) and then with nifedipine. The fact that the
blood pressure fell much further than intended was probably the result of
depleted intravascular volume, which was, in turn, the result of the high
urinary output provoked by the previous diuretic and the high serum glu-
cose level. It is this last point that appears to have been unappreciated, at
first by the physicians who saw the patient initially, and then by the anesthe-
In the opinion of anesthesiologist reviewers of this incident shortly after it
occurred, the circumstances of this case should have brought to mind a series
of questions about the nature of the patient's intravascular volume. The
inability to answer those questions would then have prompted the use of
particular monitoring techniques before and during the surgical procedure.
Bringing knowledge to bear effectively in problem solving is a process
that involves issues of knowledge
Research in this area has emphasized that mere
possession of knowledge is not enough for expertise. It is also critical for
knowledge to be organized so that it can be activated and used in different
contexts (Bransford, Sherwood, Vye, & Rieser, 1986). Thus, Feltovich,
Spiro, and Coulson (1989) and others emphasize that one component of
human expertise is the flexible application of knowledge in new situations.
There are at least four lines of overlapping research related to knowledge
use by humans in complex systems. These include (a) the role of mental
models and of knowledge flaws (sometimes called "buggy" knowledge); (b)
the issue of knowledge calibration; (c) the problem of inert knowledge; and
(d) the use of heuristics, simplifications, and approximations. In many
incidents, going behind the label "human error" demands investigating how
knowledge was or could have been brought to bear in the evolving incident.
Any of the previously mentioned factors could contribute to failures to
activate relevant knowledge in context.
Mental Models and Buggy Knowledge.
Knowledge of the world and its
operation may be complete or incomplete and accurate or inaccurate. Prac-
titioners may act based on inaccurate knowledge or on incomplete knowl-
edge about some aspect of the complex system or its operation. The term
mental model
has been used to describe the collection of knowledge used by
a practitioner. When the mental model is inaccurate or incomplete, its use can
give rise to inappropriate actions. These mental models are described as
"buggy" (see Chi, Glaser, & Farr,1988; Gentner & Stevens, 1983; and Rouse
& Morris, 1986, for some of the basic results on mental models). Studies of
practitioners' mental models have examined the models that people use for
understanding technological, physical, and physiological processes.
For example, Sarter and Woods (1992, 1994) found that buggy mental
models contributed to the problems pilots experienced in using cockpit
automation. Airplane cockpit automation has various modes of automatic
flight control, ranging between the extremes of automatic and manual. The
modes interact with each other in different flight contexts. Having a detailed
and complete understanding of how the various modes of automation inter-
act and the consequences of transitions between modes in various flight
contexts is a demanding new knowledge requirement for the pilot in highly
automated cockpits. They also found that buggy mental models played a
role in automation surprises, cases where pilots are "surprised" by the
automation's behavior. The buggy knowledge contributed to difficulties in
monitoring and understanding automatic system behavior (What is it doing?
Why did it do that?) and to projecting or anticipating future states (What
will it do next?). This is a common finding in complex systems and has also
been described in anesthesiologists using microcomputer-based devices
(Cook, Potter,
Woods, & McDonald, 1991). Significantly, the design of
devices, particularly the interface between the device and human practitio-
ners, can either aid or impede the development of useful mental models by
practitioners. The presence of a buggy mental model of a device is more
likely to indicate poor device design than it is some inadequacy of the user's
mental machinery (Norman, 1988).
It is possible to design experiments that reveal specific bugs in practitio-
mental models. By forcing pilots to deal with various nonnormal
situations, it was possible to reveal gaps or errors in their understanding of
how the automation works in various situations. Although pilots were able
to make the automation work in typical flight contexts, they did not fully
exploit the range of the system's capabilities. Pilots tend to adopt and stay
with a small repertoire of strategies, in part because their knowledge about
the advantages and disadvantages of the various options for different flight
contexts is incomplete. In unusual or novel situations, however, it may be
essential to have a thorough understanding of the functional structure of the
automated systems and to be able to use this knowledge in operationally
effective ways.
Novel or unusual situations can reveal the presence of a "buggy" mental
model, and many incidents are associated with situations that are unusual to
some degree. It can be quite difficult to determine whether a buggy mental
model was, indeed, involved in an incident. In the exemplar incident, for
example, the combination of congestive heart failure (normally improved by
reducing the amount of fluid in the circulation) with high urine output from
high blood glucose and a diuretic drug (furosemide) was unusual. It is not
clear whether the practitioner had a buggy mental model of the relationship
between these factors or if the demands of attention to the low oxygen
saturation and blood pressure prevented him from examining the model
closely enough to discover the relationship. Alternatively, the mental model
and associated knowledge may simply have been inert (see the section on
inert knowledge). The inability to distinguish between these alternatives is
due, in large part, to the limitations of the data about the incident.
Knowledge Calibration.
Results from several studies (Cook, Potter,
Woods, & McDonald, 1991; Moll van Charante, Cook, Woods, Yue, &
Howie, 1993; Sarter & Woods, 1994) indicate that practitioners may be
unaware of gaps or bugs in their model of a device or system. This raises the
question of knowledge calibration (Wagenaar & Keren, 1986). Everyone has
some areas where their knowledge is more complete and accurate than
others. Individuals are well calibrated if they are aware of how well they know
what they know. People are miscalibrated if they are overconfident and
believe that they understand areas where in fact their knowledge is incom-
plete or buggy?
There are several factors that could contribute to miscalibration of prac-
titioners' awareness about their knowledge of the domain and the technol-
ogy with which they work. First, areas of incomplete or buggy knowledge
can remain hidden from practitioners because they have the capability to
work around these areas by sticking with a few well-practiced and well-
understood methods. Second, situations that challenge practitioner mental
models or force them to confront areas where their knowledge is limited and
miscalibrated may arise infrequently. Third, studies of calibration have
indicated that the availability of feedback, the form of feedback, and the
attentional demands of processing feedback can effect knowledge calibra-
tion (e.g.,
Wagenaar & Keren, 1986).
Problems with knowledge calibration can be severe, especially when
information technology is involved in practice. For example, many comput-
erized devices fail to provide adequate feedback to users to allow them to
learn about (to calibrate) the internal relationships of the device. A relation-
between poor feedback and miscalibrated practitioners was found in
studies of pilot-automation interaction (Sarter & Woods, 1994) and of
physician-automation interaction (Cook, Potter,
Woods, & McDonald,
1991). For example, some of the participants in the former study made
comments in the postscenario debriefings such as: "I never knew that I did
not know this. I just never thought about this situation." Although this is
phenomenon is most easily demonstrated when practitioners attempt to use
computerized devices, it is probably ubiquitous.
One physician was recently heard to describe another as being "often wrong but never in
doubt," an indication that practitioners may recognize the presence of a calibration problem.
Activating Relevant Knowledge in Context The Problem of Inert
Lack of knowledge or buggy knowledge may be one part of the
puzzle, but the more critical question may be factors that affect whether
relevant knowledge is activated for use in the actual problem-solving context
(e.g., Bransford et al, 1986). The question is not just whether the problem
solver knows some particular piece of domain knowledge, but whether he or
she calls it to mind when it is relevant to the problem at hand and whether he
or she knows how to use this knowledge in problem solving. We tend to
assume that if a person can be shown to possess a piece of knowledge in any
circumstance, then this knowledge should be accessible under all conditions
where it might be useful. In contrast, a variety of research has revealed
dissociation effects where knowledge accessed in one context remains inert
in another (Gentner & Stevens, 1983; Perkins & Martin, 1986). This situation
may well have been the case in the first incident: The practitioner knew about
the relationships determining the urine output in the sense that he was able
to explain the relationships after the incident, but this knowledge was inert,
that is, it was not summoned up during the incident.
The fact that people possess relevant knowledge does not guarantee that
this knowledge will be activated when needed. The critical question is not to
show that the problem solver possesses domain knowledge as might be
determined by standardized tests. Rather, the more stringent criterion is
that situation-relevant knowledge is accessible under the conditions in which
the task is actually performed. Thus,
inert knowledgeis knowledge acces-
sible only in a restricted set of contexts, which may not include contents of
relevance to actual practice. Inert knowledge may be related to cases that
are difficult not because problem solvers do not know the individual pieces
of knowledge needed to build a solution, but because they have not previ-
ously confronted the need to join the pieces together. Thus, the practitioner
in the first incident could be said to
about the relationship between
blood glucose, furosemide, urine output, and intravascular volume but also
not to know
about that relationship in the sense that the knowledge was not
activated at the time when it would have been useful. Studies of practitioner
interaction with computerized systems show that the same pattern can occur
with computer aids and automation. Sarter and Woods (1994) found that
some pilots clearly possessed knowledge because they were able to recite
the relevant facts in debriefing, but they were unable to apply the same
knowledge successfully in an actual flight context; that is, their knowledge
was inert.
Results from accident investigations often show that the people involved
did not call to mind all the relevant knowledge during the incident although
they "knew" and recognized the significance of the knowledge afterwards.
The triggering of a knowledge item X may depend on subtle pattern recog-
nition factors that are not present in every case where X is relevant. Alterna-
tively, that triggering may depend critically on having sufficient time to
process all the available stimuli in order to extract the pattern. This may
explain the difficulty practitioners have in "seeing" the relevant details in a
certain case where the pace of activity is high and there are multiple de-
mands on the practitioner. These circumstances were present in Incident #1
and are typical of systems "at the edge of the performance envelope."
Heuristics, Simplifications and the Imprecision of Knowledge.
the past decade, there has been much written about medical decision making,
and a large portion of it is highly critical of the decision processes of
practitioners. People tend to cope with complexity through simplifying
heuristics, that is, through rules of thumb and simplifications. Heuristics are
because they are usually relatively easy to apply and minimize the
cognitive effort required to produce decisions. Heuristics can readily be
shown to be incorrect under some circumstances (Tversky & Kahneman,
1974)' and, in theory, are less desirable as decision rules than precise
computations, at least if the decision maker is considered to have infinite
mental resources for computation. However, these simplifications may also
be useful approximations that allow limited-resource practitioners to func-
tion robustly over a variety of problem demand factors (Woods, 1988).
At issue is whether a simplification is (a) generally useful because it
reduces mental workload without sacrificing accuracy, or (b) a distortion or
misconception that appears to work satisfactorily under some conditions but
leads to error in others. The latter class is described by Feltovich et al. (1989)
as anoversimplification.
In studying the acquisition and representation of
complex concepts in biomedicine, Feltovich et al. found that various over-
simplifications were held by some medical students and even by some
practicing physicians. They found that "bits and pieces of knowledge, in
themselves sometimes correct, sometimes partly wrong in aspects, or some-
times absent in critical places, interact with each other to create large-scale
and robust misconceptions" (Feltovich et al., 1989, p. 162). Examples of
kinds of oversimplification include:
Seeing different entities as more similar than they actually are.
Treating dynamic phenomena statically.
Assuming that some general principle accounts for all of a phenom-
Treating multidimensional phenomena as unidimensional or accord-
ing to a subset of the dimensions.
Treating continuous variables as discrete.
'Indeed, if a rule of thumb is not inaccurate in some circumstance then it is a robust rule and
not a heuristic at all.
Treating highly interconnected concepts as separable.
Treating the whole as the sum of its parts (see Feltovich, Spiro, &
Coulson, 1993).
These oversimplifying tendencies may occur because of requirements for
cognitive effort in demanding circumstances.
It is easier to think that all instances of the same nominal concept ... are the
same or bear considerable similarity. It is easier to represent continuities in
terms of components and steps. It is easier to deal with a single principle from
which an entire complex phenomenon "spins out" than to deal with numerous,
more localized principles and their interactions. (Feltovich et al., 1989, p. 131)
Criticisms of practitioner decision making based on simplified or over-
simplified knowledge are often used to show that practitioners make bad
decisions and that their decision making would be improved by adopting a
more mathematically rigorous, probabilistic reasoning approach. It can be
shown mathematically, for example, that a particular strategy for contingent
choice using strict criteria would be preferable to many other strategies.
Such demonstrations are usually sterile exercises, however, for several rea-
sons. First, the effort required to perform such calculations may be so large
that it would keep practitioners from acting with the speed demanded in
actual environments. This has been shown elegantly by Payne and col-
leagues (Payne, Bettman, & Johnson, 1988; Payne, Johnson, Bettman, &
Coupey, 1990) who demonstrated that simplified methods will produce a
higher proportion of correct choices between multiple alternatives under
conditions of time pressure. Put simply, if the time and effort required to
arrive at a decision is important, it may be possible to have an overall higher
quality performance using heuristics than using a "mathematically ideal"
The second reason that it is difficult to rely on formal decision making
methods is that medical knowledge is so heterogeneous and imprecise.
Much medical research data are drawn from small or only marginally repre-
sentative samples; drug tests rarely include pregnant women, for example,
so the effects of many drugs on pregnant women and fetuses are unknown.
Much patient data are derived from coarse measurements at widely spaced
intervals, whereas others (for example, the effects of exposure to anesthetic
agents) are known precisely but only for a limited period of time. Thus it is
possible to have quite precise knowledge about the effect of a disease or a
treatment on a specific subset of patients and also to have a great deal of
uncertainty about the extent to which that knowledge is useful for a given
patient both because the knowledge is derived from a specific subgroup and
because the patient is poorly characterized. Many important physiologic
Conflicting domain knowledge. For a cardiac surgery patient the blood
pressure should be kept low to minimize the work of the heart, but the blood pressure
should be kept high to maximize the blood flow to heart muscle. Flow practitioners at the
sharp end resolve this conflict depends on several factors (from Cook, Woods, &
reprinted by permission).
change without therapy? How long will the surgery last? What is the level of
surgical skill being employed? As is often the case in this and similar
domains, the locus of conflict may vary from case to case and from moment
to moment. It is impossible to create algorithms that adequately capture the
variety of patient characteristics and risks in a highly uncertain world. These
conflicts are a normal part of the medical domain and practitioners are so
comfortable with them that it is hard to get the participants in an incident to
be explicit about the trade-offs involved in the decisions they made.
In summary, heuristics may represent effective and necessary adaptations
to the demands of real workplaces (Rasmussen, 1986). The problem, if there
is one, may not always be the shortcut or simplification itself, but whether
practitioners know the limits of the shortcuts, can recognize situations where
the simplification is no longer relevant, and have the ability to use more
complex concepts, methods, or models (or the ability to integrate help from
specialist knowledge sources) when the situation they face demands it.
Interestingly, practitioners are acutely aware of how deficient their rules of
thumb may be and how certain situations may require abandoning the
cognitively easy method in favor of more cognitively demanding "deep
thinking." For example, senior anesthetists commenting on the practitioner's
behavior in the first incident were critical of his performance:
This man was in major sort of hyperglycemia and with popping in extra Lasix
[furosemide] you have a risk of hypovolemia from that situation. I don't
understand why that was quietly passed over, I mean that was a major emer-
gency in itself.... This is a complete garbage amount of treatment coming in
from each side, responding from the gut to each little bit of stuff [but it] adds up
to no logic whatsoever.... The thing is that this patient [had] an enormous
number of medical problems going on which have been simply reported [but]
haven't really been addressed.
This critique is not quite correct. In fact, each problem was addressed in
some way at some time. But the comment about "coming in from each side"
identifies what the practitioner was missing in the incident, that is, the
interactions between normally separate factors that here were closely linked.
Being able to discover that link and appreciate its implications is intimately
bound up with knowledge factors including mental models, heuristics, and
inert knowledge.
Attentional Dynamics: Incident #2-Hypotension
During a coronary artery bypass graft procedure, an infusion controller device
delivered a large volume of a potent drug to the patient at a time when no drug
should have been flowing. Five of these microprocessor-based devices were set
up in the usual fashion at the beginning of the day, prior to the beginning of the
variables can be measured only indirectly with poor precision and are
known to fluctuate widely even in the healthy population. Physicians often
must rely on comparatively remote or indirect measures of critical variables.
The precise effect of a therapy is usually only predictable for a group of
patients; for example, a preoperative antibiotic will reduce the risk of
postoperative infection by a small amount, but the actual benefit to an
individual patient coming to the operating room for a specific procedure is
extraordinarily difficult to define. All these factors tend to lead medical
practitioners toward an empirically based approach to diagnosis and therapy
in which successive treatments are applied until the desired result is achieved.
There are also inherent conflicts in the knowledge base that need to be
resolved in each individual case by the practitioner. In Incident #1, for
example, there are conflicts between the need to keep the blood pressure
high and the need to keep the blood pressure low (Fig. 13.2). The heart
depends on blood pressure for its own blood supply, but increasing the
blood pressure also increases the work it is required to perform. The practi-
tioner must decide what blood pressure is acceptable. Many factors enter
into this decision process: What is the patient's normal blood pressure? How
labile is the blood pressure now? How will attempts to reduce blood pres-
sure affect other physiological variables? How is the pressure likely to
case. The initial sequence of events associated with the case was unremark-
able. Elevated systolic blood pressure (>160 tort) at the time of sternotomy
prompted the practitioner to begin an infusion of sodium nitroprusside via one
of the devices. After this device was started at a drop rate of 10/min, the device
began to sound an alarm. The tubing connecting the device to the patient was
checked and a stopcock (valve) was found to be closed. The operator opened
the stopcock and restarted the device. Shortly after restart, the device alarmed
again. The blood pressure was falling by this time, and the operator turned the
device off. Over a short period, hypertension gave way to hypotension (sys-
tolic pressure <60 tort). The hypotension was unresponsive to fluid challenge
but did respond to repeated boluses of neosynephrine and epinephrine. The
patient was placed on bypass rapidly. Later, the container of nitroprusside was
found to be empty; a full bag of 50 mg in 250 ml was set up before the case.
The physicians involved in the incident were comparatively experienced
device users. Reconstructing the events after the incident led to the conclu-
sion that the device was assembled in a way that would allow free flow of
drug. Drug delivery was blocked, however, by a closed downstream stop-
cock. The device was started, but the machine did not detect any flow of drug
(the stopcock was closed), triggering visual and auditory alarms. When the
stopcock was opened, free flow of fluid containing drug began. The control-
ler was restarted, but the machine again detected no drops because the flow
was wide open and no individual drops were formed. The controller alarmed
again, with the same message, which appeared to indicate that no flow had
occurred. Between the opening of the stopcock and the generation of the
error message, sufficient drug was delivered to substantially reduce the
blood pressure. The operator saw the reduced blood pressure, concluded
that the sodium nitroprusside drip was not required, and pushed the button
marked "off." This powered down the device, but the flow of drug contin-
ued. The blood pressure fell even further, prompting a diagnostic search for
sources of low blood pressure. The sodium nitroprusside controller was seen
to be off. Treatment of the low blood pressure itself commenced and was
successful. The patient suffered no sequelae'
This incident is used as an exemplar for the discussion of attentional
dynamics, although it also involves a number of issues relevant to knowledge
Attentional dynamics refers to those factors affecting cognitive
function in dynamic evolving situations, especially those involving the man-
agement of workload in time and the control of attention when there are
multiple signals and tasks competing for a limited attentional focus. In many
ways, this is the least explored frontier in cognitive science, especially with
This case is described more fully in Cook, Woods, and Howie (1992), and weaknesses in the
infusion device from the point of view of human-computer cooperation are covered in Moll van
Charante et al. (1993).
respect to error. In dynamic, event-driven environments like the operating
room, attentional factors are often crucial in the evolution of incidents (cf.
Gopher, 1991; Hollister, 1986; Woods, 1992).
In Incident #2, the data are strong enough to support a reconstruction of
some of the actual changes in focus of attention of the participants during
the incident. A collection of infusion devices like those involved in the
incident are shown in Fig. 13.3. The free flow of the drug began when one of
the physicians opened the stopcock downstream of the affected device, but
this source of the hypotension was not identified until the bag of fluid was
nearly empty. There are a number of factors in the environment that
contributed to the failure to observe (i.e., attend to) the unintended flow of
drug via the infusion device, including: (a) the drip chamber being obscured
by the machine's sensor, making visual inspection difficult, (b) presence of
an aluminum shield around the fluid bag, hiding its decreasing size, (c)
misleading alarm messages from the device, and (d) presence of multiple
devices, making it difficult to trace the tubing pathways.
There are also extra-environmental factors that contributed to the failure
to observe the free flow. Most importantly, the practitioners reported that
they turned the device off as soon as the pressure fell and the device alarmed
a second time. In their view of the external world, the device was off,
therefore not delivering any drug, and therefore not a plausible source of the
hypotension. When they looked at the device, the displays and alarm mes-
sages indicated that the device was not delivering drug or later that it had
been turned off. The issue of whether off might have meant something else
(e.g., that the device was powered down but a path for fluid flow remained
open) might have been revisited had the situation been less demanding, but
the fall in blood pressure was a critical threat to the patient and demanded
the limited resource of attention. Remarkably, the practitioners intervened
in precisely the right way for the condition they were facing. The choice of
drug to increase the blood pressure was ideal to counteract the large dose of
sodium nitroprusside that the patient was receiving. Attention did not focus
on the fluid bags on the infusion support tree until the decision was made to
start an infusion of the antagonist drug and a bag for that drug was being
placed on the support tree.
This incident is remarkable, in part for the way in which it shows both the
fragility and robustness of human performance. The inability to diagnose
the cause of hypotension is in contrast to the ability to manage successfully
the complications of the inadvertent drug delivery. There are a number of
potential causes of hypotension in the cardiac surgery patient. In this case,
successful diagnosis of the cause was less important than successful treat-
ment of the consequences of the problem. The practitioners were quick to
correct the physiologic, systemic threat even though they were unable to
diagnose its source. This shift from diagnosis to what Woods (1988, 1994)
A set-up of multiple drug Infusion devices in the heart room. Drugs to raise
and lower blood pressure and other cardiovascular system parameters are In the fluid
bags above. The controller boxes regulate flow through the tubing based on the
detection of fluid drops in drip chambers connected to the bags. The Individual flows are
joined together by a series of stopcocks to a single piece of tubing, which is then
connected to the patient. (See Moll van Charente et al., 1993, for additional details.)
disturbance management
is crucial in the operating room and in other
domains to maintaining the system in a stable configuration to permit later
diagnosis and correction of the underlying faults.
The control of attention is an important issue for those trying to under-
stand human performance, especially in event-rich domains such as the
operating room. Attention is a limited resource. One cannot attend to more
than one thing at a time, and so shifts of attention are necessary to be able to
"take in" the ways in which the world is changing. When something in the
world is found that is anomalous (what is sensed in the world is not consis-
tent with what is expected by the observer), attention focuses on that thing,
and a process of investigation begins that involves other shifts of attention.
This process is ongoing and has been described by Neisser as the
(Neisser, 1976; Tenney, Jager Adams, Pew, Huggins, & Rogers, 1992).
It is a crucial concept for those trying to understand human performance
because it is the basis for all diagnosis and action. Nothing can be discovered
in the world without attention; no intended change in the world can be
effected without shifting attention to the thing being acted upon. At least
two major human performance problems can arise from alterations in
attentional dynamics. The first is a loss of situation awareness, and the
second is psychological fixation.
Loss of Situation Awareness.
Situation awareness is a label that is often
used to refer to many of the cognitive processes involved in attentional
dynamics (Sarter & Woods, 1991; Tenney et al., 1992). Just a few of the
cognitive processes that may pass under the label of situation awareness are:
control of attention(Gopher, 1991),
mental simulation
(Klein & Crandall, in
directed attention
Woods, 1992), and
contingency planning
1990). Because the concept involves tracking processes in time, it has also
been described as
mental bookkeepingto track multiple threads of different
but interacting subproblems (Cook, Woods, & McDonald, 1991; Dorner,
1983). These terms refer to tracking the shifting pattern of interactions in the
system under control. For example, the state of chemical paralysis of the
patient and the "depth" of anesthesia are two different threads. Normally
these may be treated independently, but under some circumstances they may
interact in ways that have implications for the future course of the patient.
Maintaining situation awareness necessarily requires shifts of attention
between the various threads. It also requires more than attention alone, for
the object of the shifts of attention is to inform and modify a coherent
picture or model of the system as a whole. Building and maintaining that
picture requires cognitive effort.
Breakdowns in these cognitive processes can lead to operational difficul-
ties in handling the demands of dynamic, event-driven incidents. In aviation
circles, this is known as "falling behind the plane," and in aircraft carrier
flight operations it has been described as "losing the bubble" (Roberts &
Rousseau, 1989). In each case what is being lost is some of the operator's
internal representation of the state of the world at that moment and the
direction in which the forces active in the world are taking the system that
the operator is trying to control.
Obtaining a clear, empirically testable model for situation awareness is
difficult. For example, Hollister (1986) presented an overview of a model of
divided attention operations-tasks where attention must be divided across
a number of different input channels and where the focus of attention
changes as new events signal new priorities. This model then defines an
approach to breakdowns in attentional dynamics (what has been called a
divided attention theory of error) based on human divided attention capa-
bilities balanced against task demands and adjusted by fatigue and other
performance-shaping factors. Situation awareness is clearly most in jeop-
ardy during periods of rapid change and where a confluence of forces makes
an already complex situation critically so. This condition is extraordinarily
difficult to reproduce convincingly in a laboratory setting. Practitioners are,
however, particularly sensitive to the importance of situation awareness,
even though researchers find that a clear definition remains elusive (Sarter
& Woods, 1991).
Failures to Revise Situation Assessments: Fixation or Cognitive
The results of several studies (Cook, McDonald, & Smallhout,
1989; De Keyser & Woods, 1990; Gaba & DeAnda, 1989; Johnson, Moen, &
Thompson, 1988; Johnson & Thompson, 1981; Woods, O'Brien, & Hanes,
1987) strongly suggest that one source of error in dynamic domains is
a failure
to revise
situation assessment as new evidence comes in. Evidence discrepant
from the agent's or team's current assessment is missed or discounted or
rationalized as not really being discrepant with the current assessment. In
addition, it seems that several major accidents involved a similar pattern of
behavior from the operational teams involved; examples include the Three
Mile Island accident (Kemeny et al., 1979) and the Chernobyl accident.
Many critical real-world human problem solving situations take place in
dynamic, event-driven environments where the evidence arrives over time
and situations can change rapidly. Incidents rarely spring, full blown and
complete; incidents
In these situations, people must amass and
integrate uncertain, incomplete, and changing evidence; there is no single
well-formulated diagnosis of the situation. Rather, practitioners make pro-
visional assessments based on partial and uncertain data. These assessments
are incrementally updated and revised as more evidence comes in. Further-
more, situation assessment and plan formulation are not distinct sequential
stages, but rather they are closely interwoven processes with partial and
provisional plan development and feedback leading to revised situation
assessments (Klein, Orasanu, Calderwood, & Zsambok, 1993; Woods &
Roth, 1988).
In psychological fixations, the initial situation a
zssment tends to be
appropriate, in the sense of being consistent with the partial information
available at that early stage of the incident. As the incident evolves, how-
ever, people fail to revise their assessments in response to new evidence,
evidence that indicates an evolution away from the expected path. The
practitioners become fixated on an old assessment and fail to revise their
situation assessment and plans in a manner appropriate to the data now
present in their world.
A fixation
occurs when practitioners fail to revise
their situation assessment or course of action and maintain an inappropriate
judgment or action
in the face of opportunities to revise.
Several criteria are necessary to describe an event as a fixation. One
critical feature is that there is some form of
over time in the
behavior of the fixated person or team. Second, opportunities to revise are
cues, available or potentially available to the practitioners, that could have
started the revision process if observed and interpreted properly. In part,
this feature distinguishes fixations from simple cases of lack of knowledge or
other problems that impair error detection and recovery (Cook et al., 1989)s
The basic defining characteristic of fixations is that the immediate problem-
solving context has biased the practitioners in some direction. In naturally
occurring problems, the context in which the incident occurs and the way the
incident evolves activate certain kinds of knowledge as relevant to the
evolving incident. This knowledge in turn affects how new incoming infor-
mation is interpreted. After the fact or after the correct diagnosis has been
pointed out, the solution seems obvious, even to the fixated person or team.
De Keyser and Woods (1990) describe several patterns of behavior that
have been observed in cases of practitioner fixation. In the first one, "every-
thing but that," the practitioners seem to have many hypotheses in mind, but
never entertain the correct one. The external behavior looks incoherent
because they are jumping from one action to another without any success.
The second pattern of behavior is the opposite: "this and nothing else." The
practitioners are stuck on one strategy, one goal, and they seem unable to
shift or to consider other possibilities. The persistence in practitioner behav-
ior can be remarkable. For example, practitioners may repeat the same
action or recheck the same data channels several times. This pattern is easily
identified because of the unusual number of repetitions despite an absence
Of course, the interpretation problem is to define a standard to use to determine what cue
or when a cue should alert the practitioners to the discrepancy between the perceived state of the
world and the actual state of the world. There is a great danger of falling into the hindsight bias
when evaluating after the fact whether a cue "should" have alerted the problem solvers to the
of results. The practitioners often detect the absence of results themselves
but without any change in strategy. A third pattern is "everything is OK"
(Perrow, 1984). Here the practitioners do not react to the change in their
environment. Even if there are a lot of cues and evidence that something is
going wrong, they do not seem to pay much attention to them. The practitio-
ners seem to discount or rationalize away indications that are discrepant
with their model of the situation.
There are certain types of problems that may encourage fixations by
mimicking other situations. This, in effect,
leads practitioners down
a garden
In garden path problems,
"early cues strongly suggest [plausible but]
incorrect answers, and later, usually weaker cues suggest answers that are
correct" (Johnson et al., 1988). It is important to point out that the errone-
ous assessments resulting from being led down the garden path are not due
to knowledge factors. Rather, they seem to occur because "a problem
solving process that works most of the time is applied to a class of problems
for which it is not well suited" (Johnson et al., 1988). This notion of garden
path situations is important because it identifies a task genotype in which
people become susceptible to fixations. The problems that occur are best
attributed to the interaction of particular environmental (task) features and
the heuristics people apply (locally rational strategies given difficult prob-
lems and limited resources), rather than to the any particular bias or prob-
lem in the strategies used. Simply put, going down a garden path is not an
"error" per se. It is how the problem presents to practitioners that makes it
easy to entertain plausible but erroneous possibilities. Anesthesiology and
similar domains have inherent uncertainties in diagnostic problems, and it
may be necessary for practitioners to entertain and evaluate what turn out to
be erroneous assessments. Problems arise when the revision process breaks
down and the practitioner becomes fixated on an erroneous assessment,
missing, discounting or reinterpreting discrepant evidence (see Johnson et
al., 1988; Roth, Woods, & Pople, 1992, for analyses of performance in
garden path incidents). What is important is the process of "error" detection
and recovery, which fundamentally involves searching out and evaluating
discrepant evidence in order to keep up with a changing incident.
Fixation is a characteristic of practitioners in an incident. There are
several cognitive processes involved in attentional dynamics that may give
rise to fixation:
Breakdowns in shifting or scheduling attention as the incident un-
2. Factors of knowledge organization and access that make critical
knowledge inert.
Difficulties calling to mind alternative hypotheses that could account
for observed anomalies-problems in the processes underlying hy-
pothesis generation.
Problems in strategies for situation assessment (diagnosis) given
probability of
factors, for example, how to value parsimony
(single-factor assessments) versus multifactor interpretations.
Fixation may represent the down side of normally efficient and reliable
cognitive processes involved in diagnosis and disturbance management in
dynamic contexts. Although fixation is fundamentally about problems in
attentional dynamics, it may also involve inert knowledge (calling to mind
potentially relevant knowledge such as alternative hypotheses) or strategic
factors (trade-offs about what kinds of explanations to prefer).
It is clear that in demanding situations where the condition of the patient
and the operating room system is changing rapidly, there is a potential
conflict between the need to revise the situation assessment and the need to
maintain coherence. Not every change is important; not every signal is
meaningful. The practitioner whose Attention is constantly shifting from one
item to another may not be able to formulate a complete and coherent
picture of the state of the system. For example, the practitioner in Incident
#1 was criticized for failing to build a complete picture of the patient's
changing physiological state. Conversely, the practitioner whose attention
does not shift may miss cues and data that are critical to updating the
situation assessment. This latter condition may lead to fixation. How practi-
tioners manage this conflict is largely unstudied.
Strategic Factors: Incident
Weekend Operating
On a weekend in a large tertiary care hospital, the anesthesiology team
(consisting of four physicians of whom three are residents in training) was
called on to perform anesthetics for an in vitro fertilization, a perforated
viscus, reconstruction of an artery of the leg, and an appendectomy, in one
building, and one exploratory laparotomy in another building. Each of these
cases was an emergency, that is, a case that cannot be delayed for the regular
daily operating room schedule. The exact sequence in which the cases were
done depended on multiple factors. The situation was complicated by a de-
manding nurse who insisted that the exploratory laparotomy be done ahead of
other cases. The nurse was only responsible for that single case; the operating
room nurses and technicians for that case could not leave the hospital until the
case had been completed. The surgeons complained that they were being
delayed and their cases were increasing in urgency because of the passage of
time. There were also some delays in preoperative preparation of some of the
patients for surgery. In the primary operating room suites, the staff of nurses
and technicians were only able to run two operating rooms simultaneously.
The anesthesiologist in charge was under pressure to attempt to overlap
portions of procedures by starting one case as another was finishing so as to
use the available resources maximally. The hospital also served as a major
trauma center, which means that the team needed to be able to start a large
emergency case with minimal (less than 10 minutes) notice. In committing all
of the residents to doing the waiting cases, the anesthesiologist in charge
produced a situation in which there were no anesthetists available to start a
major trauma case. There were no trauma cases, and all the surgeries were
accomplished. Remarkably, the situation was so common in the institution
that it was regarded by many as typical rather than exceptional.
The third incident is remarkable in part because it is regarded as unre-
markable by the participants. These kinds of scheduling issues recur and are
considered by many to be simply part of the job. In the institution where the
incident occurred, the role of being anesthetist in charge during evening and
weekend duty is to determine which cases will start and which ones will wait.
Being in charge also entails handling a variety of emergent situations in the
hospital, including calls to intubate patients on the floor, requests for pain
control, and emergency room trauma cases. The in-charge person also
serves as a backup resource for the operations in progress. In this incident,
the anesthetist in charge committed all of her available resources, including
herself, to doing anesthesia. This effectively eliminated the in-charge person's
ability to act as a buffer or extra resource for handling an additional trauma
case or a request from the floor. There were strong incentives to commit the
resources, but also a simultaneous incentive to avoid that commitment.
Trauma severe enough to demand immediate surgery occurs in this institu-
tion once or twice a week.
Factors that played a role in the anesthetist's decision to commit all
available resources included the relatively high urgency of the cases, the
absence of a trauma alert (indication that a trauma patient was in route to
the hospital), the time of day (fairly early; most trauma is seen in the late
evening or early morning hours), and the pressure from surgeons and
nurses. Another seemingly paradoxical reason for committing the resources
was the desire to free up the resources by getting the cases completed before
the late evening when trauma operations were more likely. These factors are
not severe or even unusual. Rather, they represent the normal functioning
of a large urban hospital as well as the nature of the conflicts and double
binds that occur are part of the normal playing field of the specialty.
The conflicts and their resolution presented in Incident #3 and the trade-
offs between highly unlikely but highly undesirable events and highly likely
but less catastrophic ones are examples of strategic factors. People have to
make trade-offs between different but interacting or conflicting goals, be-
tween values or costs placed on different possible outcomes or courses of
action, and between the risks of different errors. People make these trade-
offs when acting under uncertainty, risk, and the pressure of limited re-
sources (e.g., time pressure, opportunity costs). One may think of these
trade-offs in terms of simplistic global examples like safety versus economy.
Trade-offs also occur on other dimensions. In dynamic fault management,
for example, there is a trade-off with respect to when to commit to a course
of action. Practitioners have to decide whether to take corrective action
early in the course of an incident with limited information, or to delay the
response to wait for more data to come in, to search for additional findings,
or to ponder additional alternative hypotheses. Practitioners also trade-off
between following operational rules or taking action based on reasoning
about the case itself (cf. Woods et al., 1987). Do the standard rules apply to
this particular situation when some additional factor is present that compli-
cates the textbook scenario? Should we adapt the standard plans, or should
we stick with them regardless of the special circumstances? Strategic trade-
offs can also involve coordination among agents in the distributed human-
machine cognitive system (Roth, Bennett, & Woods, 1987). A machine
expert recommends a particular diagnosis or action, but your own evalua-
tion is different.
What is enough evidence that the machine is wrong to
justify disregarding the machine expert's evaluation and proceeding on your
own evaluation of the situation? The pulse oximeter may provide an unreli-
able reading, especially when perfusion is poor and the oxygen saturation is
low. Is the current reading of 80% indicative of an artifact or an accurate
representation of the patient's oxygen saturation?
Criterion setting on these different trade-offs may not be a conscious
process or a decision made by individuals. More likely, it may be an emer-
gent property of systems of people, either of small groups or larger organiza-
tions. The criteria may be fairly labile and susceptible to influence, or they
may be relatively stable and difficult to change. The trade-offs may create
explicit choice points for practitioners embedded in an evolving situation, or
they may cast a shadow of influence over the attentional dynamics relating
intertwined events, tasks, and lines of reasoning.
In hindsight, practitioners' choices or actions can often look to be simple
blunders. Indeed, most of the media reports of "human error in medicine"
focus on such cases. But a more careful assessment of the distributed system
including the patient, physicians, and the larger institutions comprising the
hospital may reveal strategic factors at work. Behavior in the specific inci-
dent derives from how the practitioners set their trade-off criteria across
different kinds of risks from different kinds of incidents that could occur.
Because incidents are evaluated as isolated events, such trade-offs can
appear in hindsight to be unwise or even bizarre. This is because the
individual incident is used as the basis for examining the larger system (see
later discussion of hindsight). There are many strategic factors that can be
elaborated; two forms are discussed here. The first is the presence of goal
conflicts, and the other is the responsibility-authority double bind.
Goal Conflicts.
Multiple goals are simultaneously relevant in actual
fields of practice. Depending on the particular circumstances, the means to
influence these multiple goals will interact, potentially producing conflicts
between different goals. To perform an adequate analysis of the human
performance in an evolving incident requires an explicit description of the
strategic factors acting in the incident, including the interacting goals, the
trade-offs being made, and the pressures present that shift the operating
points for these trade-offs.
The impact of potential conflicts may be quite difficult to assess. Consider
the anesthetist. Practitioners' highest level goal (and the one most often
explicitly acknowledged) is to protect patient safety. But that is not the only
goal. There are other goals, some of which are less explicitly articulated.
These goals include reducing costs, avoiding actions that would increase the
likelihood of being sued, maintaining good relations with the surgical ser-
vice, maintaining resource elasticity to allow for handling unexpected emer-
gencies, and others (Fig. 13.4).
In a given circumstance, the relationships between these goals can pro-
duce conflicts. In the daily routine, for example, maximizing patient safety
and avoiding lawsuits creates the need to maximize information about the
Conflicting Real Goals
patient through preoperative workup. The anesthetist may find some hint of
a potentially problematic condition and consider further tests that may incur
costs, risks to the patient, and a delay of surgery. The cost reduction goal
provides an incentive for a minimal preoperative workup and the use of
same-day surgery. This conflicts with the other goals (Fig. 13.4). The anes-
thetist may be squeezed in this conflict-gathering the additional informa-
tion, which in the end may not reveal anything important, will cause a delay
of surgery and decrease throughput. The delay will affect the day's surgical
schedule, the hospital and the surgeons' economic goals, and the anesthesi-
ologists' relationship with the surgeons. The external pressures for highly
efficient performance are strongly and increasingly in favor of limiting the
preoperative workup of patients and omitting tests that are unlikely to yield
mportant findings. But failing to acquire the information may reduce the ill-
defined margin of safety that exists for this patient and contribute to the
evolution toward disaster if other factors are present. Increasing external
economic pressure, in particular, can generate sharp conflicts in anesthesiol-
ogy and in other areas of medicine (Eddy, 1993a, 1993b).
For an example from outside of medicine, consider the task of en route
flight planning in commercial aviation. Pilots sometimes need to modify
their flight plans en route when conditions change (e.g., weather). Some of
the goals that need to be considered are avoiding passenger discomfort (i.e.,
avoiding turbulence), minimizing fuel expenditure, and minimizing the dif-
ference between the target arrival time and actual arrival time. Depending
on the particulars of the actual situation where the crew and dispatchers
have to consider modifying the plan, these goals can interact, requiring
prioritization and trade-offs. Layton, Smith, and McCoy (in press) created
simulated flight situations where goal conflicts arose and studied how the
distributed system of dispatchers, pilots, and computer-based advisors at-
tempted to handle these situations.
In another aviation example, an aircraft is deiced and then enters the
queue for takeoff. After the aircraft has been deiced, the effectiveness of the
deicing agent degrades with time. Delays in the queue may raise the risk of
ice accumulation. However, leaving the queue to go back to an area where
the plane can be deiced again will cause additional delays, plus the aircraft
will have to re-enter the takeoff queue again. Thus, the organization of
activities (where deicing occurs relative to queuing in the system) can create
conflicts that the practitioners must resolve because they are at the sharp
end of the system. The dilemmas may be resolved through conscious effort
by specific teams to find ways to balance the competing demands, or practi-
tioners may simply apply standard routines without deliberating on the
nature of the conflict. In either case, they may follow strategies that are
robust (but still do not guarantee a successful outcome), strategies that are
brittle (work well under some conditions but are vulnerable given other
circumstances), or strategies that are very vulnerable to breakdown. Analy-
ses of past disasters frequently find that goal conflicts played a role in the
accident evolution. For example, there have been several crashes where, in
hindsight, crews accepted delays of too great a duration and ice did contrib-
ute to a failed takeoff (Moshansky, 1992; National Transportation Safety
Board, 1993).
Goal conflicts can involve economic pressures but also intrinsic charac-
teristics of the field of activity. An example from anesthesiology is the
conflict between the desirability of a high blood pressure to improve cardiac
perfusion (oxygen supply to the heart muscle) and a low one to reduce
cardiac work (Fig. 13.2). Specific actions will depend on details of the
context. The appropriate blood pressure target adopted by the anesthetist
depends in part on the individual's strategy, the nature of the patient, kind of
surgical procedure, circumstances within the case that may change (e.g., the
risk of major bleeding), and negotiation between different people in the
operating room team (e.g., the surgeon who would like the blood pressure
kept low to limit the blood loss at the surgical site).
Constraints imposed by the organizational or social context represent
another source of goal competition. Some of the organizational factors
producing goals include management policies, legal liability, regulatory
guidelines, and economic factors. Competition between goals generated at
the organizational level was an important factor in the breakdown of safety
barriers in the system for transporting oil through Prince William Sound that
preceded the Exxon
disaster (National Transportation Safety Board,
1990). Finally, some of the goals that play a role in practitioner decision
making relate to the personal or professional interests of the people in the
operational system (e.g., career advancement, avoiding conflicts with other
It should not be thought that the organizational goals are necessarily
simply the written policies and procedures of the institution. Indeed, the
messages received by practitioners about the nature of the institution's goals
may be quite different from those that management acknowledges. Many
goals are indirect and implicit. Some of the organizational influences on how
practitioners will negotiate their way through conflicting goals may not be
explicitly stated or written anywhere. These covert factors are especially
insidious because they affect behavior and yet are unacknowledged. For
example, the Navy sent a clear message to its commanders by the differential
treatment it accorded to the commander of the
following that incident
(U.S. House of Representatives Committee on Armed Services, 1987) as
opposed to the
following that incident (Rochlin, 1991; U.S. De-
partment of Defense, 1988).
In Incident #3, economic factors, intrinsic characteristics of the domain of
practice, and organizational factors all contributed to the goal conflicts the
practitioner faced.
Expertise consists, in part, of being able to negotiate among interacting
goals by selecting or constructing the means to satisfy all sufficiently. But
practitioners may fail to deal with goal conflicts adequately. Some practitio-
ners will not follow up hints about some aspect of the patient's history
because to do so would impact the usual practices relative to throughput and
economic goals. In a specific case, that omission may turn out to be impor-
tant to the evolution of the incident. Other practitioners will adopt a defen-
sive stance and order tests for minor indications, even though the yield is
low, in order to be on the safe side. This generates increased costs and incurs
the wrath of their surgical colleagues for the delays thus generated. In either
case, the nature of the goals and pressures on the practitioner are seldom
made explicit and rarely examined critically.
In postincident analysis, in hindsight, the consequences will be apparent.
It should be clear, however, that the external pressures for highly efficient
performance are strongly in favor of limiting the preoperative workup of
patients and omitting tests that are unlikely to yield important findings.
Assessments after the incident will always identify factors that if changed
would have produced a more favorable result; large, complex systems al-
ways have many such factors available for scrutiny. Thus, if those practitio-
ner actions that are shaped by the goal conflict contribute to a bad outcome
in a specific case, then it is easy for postincident evaluations to say that a
human error occurred-the practitioners should have delayed the surgical
procedure in order to investigate the hint. The role of the goal conflict may
never be noted.
To evaluate the behavior of the practitioners involved in an incident, it is
mportant to elucidate the relevant goals, the interactions between these
goals and the factors that influenced criterion setting on how to make trade-
offs in particular situations. The role of these factors is often missed in
evaluations of the behavior of practitioners. As a result, it is easy for
organizations to produce what appear to be solutions that in fact exacerbate
conflict between goals rather than helping practitioners handle goal con-
flicts in context. In part, this occurs because it is difficult for many organiza-
tions (particularly in regulated industries) to admit that goal conflicts and
trade-off decisions arise. However distasteful to admit or whatever public
relations problems it creates, denying the existence of goal interactions does
not make such conflicts disappear and is likely to make them even tougher to
handle when they are relevant to a particular incident. As Feynman re-
marked regarding the Challenger disaster, "For a successful technology,
must take precedence over public relations, for nature cannot be
fooled" (Rogers et al., 1986, Appendix F, p. 5). The difference is that, in
medical practice, one can sweep the consequences of attempting to fool
nature under the rug by labeling the outcome as the consequence of "human
Responsibility-Authority Double Binds.
Another strategic factor that
plays a role in incidents and especially in medical practice is responsibility-
authority double binds. These are situations in which practitioners have the
responsibility for the outcome but lack the authority to take the actions they
see as necessary. Regardless of how the practitioners resolve a trade-off, from
hindsight they are vulnerable to charges of and penalties for error. In
particular, control via regimentation and bureaucratically derived policies
(just follow the procedures) or the introduction of machine-cognitive agents
that automatically diagnose and plan responses, can undermine the effective
authority of the practitioners on the scene. However, these same people may
still be responsible and held accountable both formally and informally for bad
outcomes. The results of research on the role of responsibility and authority
are limited but consistent-splitting authority and responsibility appears to
have bad consequences for the ability of operational systems to handle
variability and surprises that go beyond preplanned routines (Hirschhorn,
1993; Roth et al., 1987).
There is one important investigation of the effects of responsibility-
authority double binds in the industrial literature. Hirschhorn (1993) exam-
ined an organization's (i.e., the managers) attempts to balance the need to
adapt on line to complicating factors (relative to throughput and other
goals) with the goal of adhering absolutely strictly to written procedures. In
part this is the result of the regulatory climate that believes that absolute
adherence to procedures is the means to achieve safe operations and avoid
"human error." This creates conflicts in some situations and generates
dilemmas for the people involved. If they follow the standard procedures
strictly, the job will not be accomplished adequately; if they always wait for
formal permission to deviate from standard procedures, throughput and
productivity will degrade substantially. If they deviate and it later turns out
that there is a problem with what they did (e.g., they did not adapt ad-
equately), they may create safety or economic problems. The double bind
arises because they are held responsible for the outcome (the bad outcome,
the lost productivity, the erroneous adaptation) but do not have authority
for the work practices because they are expected to comply exactly with the
written procedures. Notice the similarity to the evolving nature of medical
practice today, with the introduction of increasing regulation and so- called
"practice parameters" (Arens, 1993).
After the Three Mile Island accident, utility managers were encouraged
by the Nuclear Regulatory Commission to develop detailed and compre-
hensive work procedures to reduce the likelihood of another major disaster.
The management at a particular nuclear power plant instituted a policy of
verbatim compliance with the procedures developed at the blunt end of the
system. However, for the people at the sharp end of the system, who actually
did things, strictly following the procedures posed great difficulties because
(a) the procedures were inevitably incomplete, contradictory, and buggy,
and (b) novel circumstances arose that were not anticipated in the written
procedures. The policy created a double bind because the people would be
wrong if they violated a procedure even though it could turn out to be an
inadequate procedure, and they would be wrong if they followed a proce-
dure that turned out to be inadequate. As Hirschhorn (1993) said:
They had much responsibility, indeed as licensed professionals many could be
personally fined for errors, but were uncertain of their authority. What free-
dom of action did they have, what were they responsible for? This gap between
responsibility and authority meant that operators and their supervisors felt
accountable for events and actions they could neither influence nor control.
Workers coped with the double bind by developing a covert work system
that involved, as one worker put it, "doing what the boss wanted, not what
he said" (Hirschhorn, 1993). There were channels for requesting changes to
the procedures, but the process was cumbersome and time-consuming. This
is not surprising: If modifications are easy and liberally granted, then it may
be seen as undermining the policy of strict procedure following. The increas-
ingly complex and bureaucratic policies and procedures of U.S. hospitals
seems likely to generate a situation similar to that described by Hirschhorn.
The n-Tuple
The three incidents that have been described are exemplars for the different
cognitive demands encountered by practitioners who work at the sharp end
of large, complex systems, including anesthetists, aircraft pilots, nuclear
power plant operators, and others. Each category of cognitive issue (knowl-
edge factors, attentional dynamics, and strategic factors) plays a role in the
conduct of anesthesia and hence plays a role in the genesis and evolution of
incidents. The division of cognitive issues into these categories provides a
tool for analysis of human performance in complex domains. The categories
are united, however, in their emphasis on the conflicts present in the do-
main. The conflicts exist at different levels and have different implications,
but the analysis of incidents depends in large part on developing an explicit
description of the conflicts and the way in which the practitioners deal with
them (Table 13.1).
Together the conflicts produce a situation for the practitioner that ap-
pears to be a maze of potential pitfalls. This combination of pressures and
goals that produce a conflicted environment for work is what we call
the n-
tuple bind.'
The practitioner is confronted with the need to follow a single
course of action from a myriad of possible courses. The choice of how to
proceed is constrained by both the technical characteristics of the domain
and the need to satisfy the "correct" set of goals at a given moment chosen
from the many potentially relevant ones. This is an example of an
overconstrained problem, one in which it is impossible to maximize the
function or work product on all dimensions simultaneously. Unlike simple
laboratory worlds with a "best" choice, real complex systems intrinsically
contain conflicts that must be resolved by the practitioners at the sharp end.
Retrospective critiques of the choices made in system operation will always
be informed by hindsight. For example, if the choice is between obtaining
more information about cardiac function or proceeding directly to surgery
with a patient who has soft signs of cardiac disease, the outcome will be a
potent determinant of the "correctness" of the decision. Proceeding with
undetected cardiac disease may lead to a bad outcome (although this is by no
means certain), but obtaining the data may yield normal results, cost money,
"waste" time, and incur the ire of the surgeon. Possessing knowledge of the
outcome, because of the hindsight bias, trivializes the situation confronting
the practitioner and makes the "correct" choice seem crystal clear.
bind is most easily seen in Incident #3, where strategic factors
dominate. The practitioner has limited resources and multiple demands for
them. There are many sources of uncertainty. How long will the in vitro
fertilization take? It should be a short case, but it may not be. The explor-
atory laparotomy may be either simple or complex. With anesthetists of
different skill levels, whom should she send to the remote location where
that case will take place? Arterial reconstruction patients usually have
associated heart disease, and the case can be demanding. Should she commit
the most senior anesthetist to that case? Such cases are also usually long, and
committing the most experienced anesthetist will tie up that resource for a
long time. What is the likelihood that a trauma case will come during the
time when all the cases will be going on simultaneously (about an hour)?
There are demands from several surgeons for their case to be the next to
Which case is the most medically important one? The general rule is
that an anesthetist has to be available for a trauma; she is herself an
anesthetist and could step in, but this would leave no qualified individual to
`This term derives from the mathematical concept of a series of numbers required to define
an arbitrary point in an n-dimensional space. The metaphor here is one of a collection of factors
that occur simultaneously within a large range of dimensions, an extension of the notion of a
double bind.
go to cardiac arrests in the hospital or to the emergency room. Is it desirable
to commit all the resources now and get all of the pending cases completed
so as to free up the people for other cases that are likely to follow?
It is not possible to measure accurately the likelihood of the various
possible events that she considers. As in many such situations in medicine
and elsewhere, she is attempting to strike a balance between common but
lower consequence problems and rare but higher consequence ones. Ex post
facto observers may view her actions as either positive or negative. On the
one hand, her actions are decisive and result in rapid completion of the
urgent cases. On the other hand, she has produced a situation where emer-
gent cases may be delayed. The outcome influences how the situation is
viewed in retrospect.
A critique often advanced in such situations is that the patient's "safety"
should outweigh all other factors and be used to differentiate between
options. Such a critique is usually made by naive individuals or administra-
tive personnel not involved in the scene. Safety is not a concrete entity, and
the argument that one should always choose the safest path (in the sense of
the path that minimizes risk to the patient) misrepresents the dilemmas that
confront the practitioner. The safest anesthetic is the one that is not con-
ducted, just as the safest airplane is the one that never leaves the ground. All
large, complex systems have intrinsic risks and hazards that must be incurred
in order to perform their functions, and all such systems have had failures.
The investigation of such failures and the attribution of cause and effect by
retrospective reviewers is discussed next.
Large, Complex System Failures: The Latent Failure Model
The spectacular failures of large, semantically complex, time-pressured,
tightly coupled, high consequence, high-reliability systems' have prompted
the study of how such systems fail and the role of human operators in
successful and unsuccessful operation. The complexity of these systems
arises in large part from the need to make them reliable. All such complex
systems include potentially disastrous failure modes and are carefully crafted
to reduce the risk of such failures. Significantly, these systems usually have
multiple redundant mechanisms, "safety" systems, and elaborate policies
'These failures include the explosion of
Apollo 13,
the destruction of the space shuttle
Herald of Free Enterprise
ferry capsizing, the Clapham Junction railroad disaster,
the grounding of the tanker
Exxon Valdez,
a number of airplane crashes, the reactor explosion
at Chernobyl, and a host of other nuclear power incidents, most particularly the destruction of
the reactor at Three Mile Island. Some of these are reviewed in Perrow (1984) and Reason (1990).
and procedures to keep them from failing in ways that produce bad out-
The results of combined operational and technical measures make sys-
tems relatively safe from single-point failures; that is, they are protected
against the failure of a single component or procedure. For example, the
routine oxygen and nitrous oxide supply for anesthesia machines is derived
from a hospital-wide pipeline. Each machine, however, has its own supply
tanks available as a backup should the hospital supply fail, as well as
elaborate valving mechanisms to insure automatic switch over to the cylin-
der supply. There are even special backups designed to shut off the flow of
nitrous oxide (which will not support life) if the oxygen pressure falls below
a preset level. In addition, the machines are gas powered and will operate
even if external electrical supplies are lost. Of course, there are components
and procedures that cannot be protected through redundancy. An example
of such a component is the nuclear power plant's reactor containment
building. The building is critical to plant safety and there is only one, but it is
lavishly constructed to withstand extreme events. Similarly, the anesthesia
machine has internal piping and mechanisms that make the machine vulner-
able to single-point failures, although these failures are few and the compo-
nents are conservatively designed (Andrews, 1990).
When large system failures do occur, they are the result of multiple,
apparently innocuous faults that occur together (Perrow, 1984; Reason,
1990; Turner, 1978). All complex systems have many such apparently minor
faults. These can include such simple items as a burned out indicator bulb on
a seldom-used control panel, a checklist that is out of date because the
corresponding equipment has been modified, or an apparently minor failure
of some backup system (for example, an emergency generator). For the
most part, the minor faults are inactive, play no role in system operation, and
are therefore described as
latent failures
(Reason, 1990). These latent fail-
ures may be found at any level within an organization from the corporate
boardroom to the individual physical components of the system. System
failures occur when a particular collection of latent failures are present
together in a combination that allows the system to fail. Rather than being
derived from the massive failure of a single component, system failures arise
from the insidious accumulation of individual faults, each of which seems
too small or unimportant to threaten the larger system. Thus
because of the combination of the brittle O-ring seals
the unex-
pectedly cold weather
the reliance on the seals in the design of the
the change in roles between the contractor and the NASA
other factors. None of these conditions was individually able
to provoke a system failure, but together they were able to disrupt an
extraordinarily safety-oriented system in a catastrophic way. In the field of
aviation, a combination of factors were responsible for the simultaneous
failure of all three engines of an L-1011 jumbo jet (Norman, 1992). In
medicine, a similar case can be found in the failure of the Therac-25 radia-
tion therapy machine. This device would, under certain highly unusual
circumstances, deliver huge doses of radiation to patients. These circum-
stances, although unlikely, did arise and injure several patients. Review of
the design and use of the Therac-25 showed that multiple small failures in
software, in testing, and in operator procedures were required to generate
the system failure (Leveson & Turner, 1993). It is important to note that the
latent failures can involve technology, operational practices, and even orga-
nizational elements: Latent failures can occur in any part of the larger
system (Reason, 1990).
These large system failures have several notable characteristics. First,
they are
Large system failures are comprised of multiple failed
components or procedures. Predicting this combination is likely to be diffi-
cult or impossible for human operators; the failure mode is hard to foresee
and prevent. Second, failures are likely to be
rather than minor.
The multiple redundancies and robust design characteristics of a large
system tend to limit small-scale failures and to minimize their consequences.
In addition, the cost of the so-called safety systems and redundancies gener-
ally encourages the development of ever larger and more economically
efficient systems in order to reduce the average cost of each unit of perfor-
mance. Thus, it is not an oil tanker accident but supertanker accident, not a
plane crash but a jumbo jet crash, not an overdose of radiation but a massive
Third, the potential for catastrophic failure encourages the employment
human skill
and expertise at the final few links in the causal chain of
events. The more delicate the system, the more important its function, the
more often a person will be charged with protecting the system's integrity or
accomplishing some critical goal. Moreover, the systems are so complex and
are operated under such variable conditions that only human operators can
be expected to have both the flexibility and judgment necessary to control
them. Fourth, because disasters are composed of a collection of latent
failures occurring together, large system failures appear in retrospect to be
unique. After-accident reviews will show that the failure depended on
having a particular pattern of small faults. As the number of latent failures
required to produce the system failure increases (i.e., as the system becomes,
in some sense, "safer"), the odds against repeating a precise pattern of
become astronomical. Paradoxically, this will make
future sys-
tem failure seem extremely unlikely, even though the accumulation of latent
failures actually makes the system more failure prone.
Gaba's group (Gaba, Maxwell, DeAnda, 1987) at Stanford suggested that
model of large system behavior and failure might apply to anesthesia
practice in particular and, by extension, to medical practice in general. He
noted that anesthesia practice includes many of the characteristics of com-
plex systems and that the infrequent anesthesia mishaps appeared to be
similar to the disasters studied in other complex systems. The anesthesiolo-
gist works in a highly complex, technologically intensive environment. The
conduct of an anesthetic is a critical process that is severely time pressured,
and the elements of the system are tightly coupled together in ways that do
not provide much slack.' Cooper's group (Cooper et al., 1978; Cooper,
Newbower, & Kitz, 1984) at Massachusetts General Hospital noted that
anesthesia incidents appeared to be unique and were difficult to analyze,
exactly as one would expect in a system that had been refined to eliminate
single-point failures. Significantly, the loci of single-point failures in the
conduct of the anesthetic have been studied and largely removed or buff-
ered by redundant components or safety procedures. Although their study
predated the latent failure model of large system failure, Cooper's data also
indicated that critical incidents that progressed toward bad outcomes re-
quired multiple, simultaneous failures. Thus, there are reasons to consider
that anesthesia practice and, by extension, modem medical practice, has the
characteristics of a large, complex system and may be expected to fail in
similar ways. A recent study supports this view (Cook, Woods, & McDonald,
One consequence of the latent failure model of large system failures is
that efforts to improve the overall system performance by "fixing" particu-
lar latent failures that contributed to a past mishap are unlikely to markedly
reduce the accident rate. Because that particular pattern of contributors is so
unlikely to recur and because there are many unrecognized latent failures
that remain in the system, the correction of one set of specific flaws is of
mited value. The usual response to a system failure is to attempt to make
certain that it doesn't happen again by producing new rules and regulations,
new equipment, and new training of key personnel. Because the exact set of
flaws is unlikely to recur, these attempts will add more cost and make the
system even more complex and brittle than it was before the accident.
Because the system is already highly reliable, some time will pass between
instituting these changes and the next accident. This leads those who pro-
mulgated the changes to believe that they have significantly improved the
system safety ("after all, it hasn't failed like that since we instituted our
program X"). After some time, however, another accident occurs, but this
time with a different sequence of events derived from a different collection
of latent failures. This apparently unique accident is seen in isolation and the
cycle is repeated (see Fig. 13.5).
'For a detailed discussion of coupling, see Perrow,1984, chap. 3 and especially Table 3.2. It
is interesting to note that Perrow does not include the operating room and anesthesia in his Fig.
3.1, although based on our studies, it would lie somewhere between aircraft and nuclear plants.
The cycle of error. Attributing system failures to human operators generates
demands for more rules, automation, and polkaing. But these actions do not significantly
reduce the number of latent failures In the system. Because overt failures are rare, a
quiet period follows Institution of these new policies, convincing administrators that the
changes have been effective. When a new overt failure occurs, it seems to be unique
and unconnected to prior failures (except In the label human am), and the cycle
repeats. With each pass through the cycle, more rules, policies, and sanctions make the
system more complicated, conflicted, and brittle, increasing the opportunities for latent
failures to contribute to disasters. (01993, R.I. Cook, reprinted by permission)
Retrospective Evaluations of Human Performance in
System Transients
Attributing System Failures to Practitioners.
System failures, near fail-
ures, and critical incidents are the usual source for investigations of human
performance. When critical incidents do occur, operator failure or human
error will almost always be indicted as a major cause of any bad outcome. In
fact, large, complex systems can be readily identified by the percentage of
critical incidents that are considered to have been caused by "human error":
The rate for these systems is typically about 70% or 75%. Incident rates
attributed to human error are the same in several domains including aviation,
nuclear power, shipping, and, most recently, in anesthesia and medicine (cf.,
Hollnagel, 1993). Cooper et al. (1978) found that anesthesiologists were
contributors in 82% of critical incidents. Wright, Mackenzie, Buchan, Cairns,
and Price (1991) and Chopra et al. (1992) found similar rates in the operating
room and intensive care unit, respectively. The repeated finding of about
three fourths of incidents arising from human error has built confidence in the
notion that there is a problem with human error in these domains. Indeed, it
is the belief that fallible humans are responsible for large system failures that
has led many system designers to use more and more technology to try to
eliminate the human operator from the system or to reduce the operator's
possible actions so as to forestall these errors.
Attributing system failure to the human operators nearest temporally
and spatially to the outcome ultimately depends on the judgment by some-
one that the processes in which the operator engaged were faulty and led to
the bad outcome. Deciding which of the many factors surrounding an
incident are important and what level or grain of analysis to apply to those
factors is the product of
processes (social and psychological pro-
cesses) of causal attribution. What we identify as the cause of an incident
depends on with whom we communicate, on the assumed contrast cases or
causal background for that exchange, and on the purposes of the inquiry
Woods et al., 1994).
For at least four reasons, it is actually not surprising that human operators
are blamed for bad outcomes. First, operators are available to blame. These
large and intrinsically dangerous systems have a few well-identified humans
at the sharp end. Those humans are closely identified with the system
function, and so it is unlikely that a bad outcome will occur without having
them present. Moreover, these individuals are charged, often formally and
institutionally, with maintaining the system's safe operation as well as the
efficient functioning of the system. For any large system failure, there will be
a human in close temporal and physical relationship to the outcome (e.g., a
ship captain, pilot, air traffic controller, physician, nurse) and available to
The second reason that human error is often the verdict after accidents is
that it is so difficult to trace backward through the causal chain that led to the
system failure (Rasmussen, 1986). It is particularly difficult to construct a
sequence that passes back through humans in the chain. To construct such a
sequence requires the ability to reconstruct, in detail, the cognitive process-
ing of operators during the events that preceded the bad outcome. There are
few tools for doing this in any but the most simple laboratory settings. The
environment of the large system makes these sorts of reconstructions ex-
tremely difficult. Indeed, a major area of research is the development of
tools to help investigators trace the cognitive processing of operators as they
deal with normal situations, situations at the edges of normality, and system
faults and failures. The incidents described in the first part of this chapter are
unusual in that substantial detail about what happened and what the partici-
pants saw and did was available to researchers. In general, most traces of
causality will begin with the outcome and work backward in time until they
encounter a human whose actions seem to be, in hindsight, inappropriate or
suboptimal. Because so little is known about how human operators process
a multitude of conflicting demands of large, complex systems (e.g., avoid
delays in the train schedule but also keep the trains from colliding), incident
analyses rarely demonstrate the ways in which the actions of the operator
made sense at the time and from their perspective.
The third reason that human error is often the verdict is paradoxical.
Human error is the attributed cause of large system accidents because
human performance in these complex systems is so good. Failures of these
systems are, by almost any measure, rare and unusual events. Most of the
system operations go smoothly; incidents that occur do not usually lead to
bad outcomes. These systems have come to be regarded as "safe" by
rather than
by control.
Those closely studying human operations in these
complex systems are usually impressed by the fact that the opportunity for
large-scale system failures is present all the time and that expert human
performance is able to prevent these failures. As the performance of human
operators improves and failure rates fall, there is a tendency to regard
system performance as a marked improvement in some underlying quality
of the system itself, rather than the honing of the operator skills and
expertise to a fine edge. The studies of aircraft carrier flight operations by
Rochlin, La Porte, and Roberts (1987) point out that the qualities of human
operators are crucial to maintaining system performance goals and that, by
most measures, failures should be occurring much more often than they do.
As consumers of these systems' products (health care, transportation, de-
fense) society is lulled by success into the belief that these systems are
intrinsically low risk and that the expected failure rate should be zero. Only
catastrophic failures receive public attention and scrutiny. The remainder of
the system operation is generally regarded as unflawed because of the low
overt failure rate, even though there are many incidents that could become
overt failures. Thorough after-accident analyses often indicate that there
were numerous incidents or "dress rehearsals" that preceded an accident, as
has been reported for the mode error at the heart of the crash of an
advanced commercial aircraft at Strasbourg (Woods et al., 1994).
This ability to trace backward with the advantage of hindsight is the
fourth major reason that human error is so often the verdict after accidents.
Hindsight bias, as Fischhoff (1975) puts it, is the tendency for people to
"consistently exaggerate what could have been anticipated in foresight."
Studies have shown consistently that people have a tendency to judge the
quality of a process by its outcome. The information about outcome biases
their evaluation of the process that was followed. After a system failure,
knowledge of the outcome biases the reviewer toward attributing failures to
system operators. During postevent reviews, knowledge of the outcome will
give reviewers the sense that participants ignored presumably obvious or
mportant factors and that the participants therefore erred. Indeed, this
effect is present even when those making the judgments have been warned
about the phenomenon and been advised to guard against it (Fischhoff,
1975,1982). Fischhoff (1982) wrote:
It appears that when we receive outcome knowledge, we immediately make
sense out of it by integrating it into what we already know about the subject.
Having made this reinterpretation, the reported outcome now seems a more
or less inevitable outgrowth of the reinterpreted situation. "Making sense" out
of what we are told about the past is, in turn, so natural that we may be
unaware that outcome knowledge has had any effect on us.... In trying to
reconstruct our foresightful state of mind, we will remain anchored in our
hindsightful perspective, leaving the reported outcome too likely looking. (p.
In effect, reviewers will tend to
the problem-solving situation
that was actually faced by the practitioner. The dilemmas facing the practi-
tioner in situ, the uncertainties, the trade-offs, the attentional demands and
double binds, all may be underemphasized when an incident is viewed in
hindsight. In complex, uncertain, highly conflicted settings, such as anesthe-
sia practice and the other similar disciplines such as military situations
(Lipshitz, 1989), critics will be unable to disconnect their knowledge of the
outcome in order to be able to make unbiased judgments about the perfor-
mance of human operators during the incident (Baron & Hershey, 1988).
Interestingly, although the phenomenon of
hindsight bias is
well known
in psychology, medical practice has had to rediscover it
de novo.
More than
a decade after Fischhoff's seminal papers, a study demonstrated the phe-
nomenon in physician judgment. Caplan, Posner, and Cheney (1990) asked
two groups of anesthesiologists to evaluate human performance in sets of
cases with the same descriptive facts but with the outcomes randomly
assigned to be either bad or neutral. The professionals consistently rated the
care in cases with bad outcomes as substandard, whereas they viewed the
same behaviors with neutral outcomes as being up to standard even though
the care (i.e., the preceding human acts) were identical. Typically, hindsight
bias in evaluations makes it seem that participants failed to account for
information or conditions that "should have been obvious" or behaved in
ways that were inconsistent with the (now known to be) significant informa-
tion. Thus, the judgment of whether or not a human error occurred is
critically dependent on knowledge of the outcome, something that is impos-
sible before the fact. Indeed,
it is clear from the studies of large system failures
'When someone claims that something "should have been obvious," hindsight bias
always present.
that hindsight bias is the greatest obstacle to evaluating the performance of
humans in complex systems after bad outcomes.
Outcome Failures and Process Defects.
It is reasonable to ask if there
are means for any evaluation of human performance in complex systems.
Indeed, the preceding argument seems a little disingenuous. On the one hand,
it is claimed that human performance is critical to the operations of complex
systems, and on the other hand, it is argued that there is no scientific way to
describe something as a human error and therefore that it is necessarily
impossible to distinguish between expert and inexpert performance.
One resolution of this apparent paradox is to distinguish between out-
come failures and process defects. Outcome failures are defined in terms of
a categorical shift in consequences on some performance dimension. Note
that outcome failures are necessarily defined in terms of the language of the
domain, for example, sequelae such as neurological deficit, reintubation,
myocardial infarction within 48 hours, or an unplanned ICU admission.
Process defects are departures from a standard about
problems should
be solved. Generally, the process defect, if uncorrected, would lead to or
increase the risk of some type of outcome failure. Process defects can be
defined in domain terms-for example, insufficient intravenous access, in-
monitoring, regional versus general anesthetic, decisions about
canceling a case, or problematic plans or actions with regard to the anes-
thetic agent of choice. They may also be defined psychologically in terms of
deficiencies in some cognitive or information-processing function-for ex-
ample, activation of knowledge in context, situation awareness, diagnostic
search, goal trade-offs.
The distinction between outcome and process is important because the
relationship between them is not fixed. Not all process defects are associated
with bad outcomes. The defect may be insufficient to create the bad out-
come by itself. In addition, as Edwards (1984) said, "a good decision cannot
guarantee a good outcome," that is, bad outcomes may result even if there
are no defects in process. This is especially true for domains such as anesthe-
siology where bad outcomes can occur despite the exercise of nearly flawless
expertise by the medical personnel involved (cf. Keats,1979,1990).
The rate of process defects may be frequent when compared with the
incidence of overt system failures. This is so because the redundant nature of
complex systems protects against many defects. It is also because the sys-
tems employ human operators whose function is, in part, to detect such
process flaws and adjust for them before they produce bad outcomes (a
process of error detection and recovery). Just such a situation can be seen in
Incident #2. Evaluating human performance by examining the process of
human problem solving in a complex system depends on specifying a start-
dard about how problems should be handled. There are several categories of
standards that can be used to evaluate defects in the process of solving a
One standard is
anormative model of task performance.
This method
requires detailed knowledge about precisely how problems should be solved,
that is, nearly complete and exhaustive knowledge of the way the system
works. Such knowledge is, in practice, rare. At best, some few components
of the larger system can be characterized in this exhaustive way. Unfortu-
nately, normative models rarely exist or are not applicable to complex
situations like anesthesia practice. Those models are largely limited to
mathematically precise situations such as games or artificial tasks in bounded
Another standard is the comparison of actual behavior to
standard oper-
ating practices (e.g.,
standards of care, policies, and procedures). These
practices are mostly compilations of rules and procedures that are accept-
able behaviors for a variety of situations. They include various protocols
(e.g., the Advanced Cardiac Life Support protocol for cardiac arrest, the
guidelines for management of the difficult airway), policies (e.g., it is the
policy of the hospital to have informed consent from all patients prior to
beginning an anesthetic), and procedures (e.g., the chief resident calls the
attending anesthesiologist to the room before beginning the anesthetic but
after all necessary preparations have been made). These standards may be
of limited value because they are either codified in ways that ignore the real
nature of the domain
or because the coding is too vague to use for
evaluation. For example, one senior anesthesiologist, when asked about the
policy of the institution regarding the care for emergent Cesarean sections
replied, "Our policy is to do the right thing." This seemingly curious phrase
in fact sums up the problem confronting those at the sharp end of large,
complex systems. It recognizes that it is impossible to comprehensively list
all possible situations and appropriate responses because the world is too
complex and fluid. Thus the person in the situation is required to account for
the many factors that are unique to that situation. What sounds like a
nonsense phrase is, in fact, an expression of the limitations that apply to all
structures of rules, regulations, and policies (cf. e.g., Roth et al., 1987;
Suchman, 1987). The set of rules is necessarily incomplete and sometimes
is not unusual, for example, to have a large body of rules and procedures that are not
followed because to do so would make the system intolerably inefficient. The "work to rule"
method used by unions to produce an unacceptable slowdown of operations is an example of the
way in which reference to standards is unrealistic. In this technique, the workers perform their
tasks to an exact standard of the existing rules and the system performance is so degraded by the
extra steps required to conform to all the rules that it becomes nonfunctional (e.g., Hirschhorn,
contradictory. It is the role of the human at the sharp end to resolve the
apparent contradictions and conflicts in order to satisfy the goals of the
In general, procedural rules are too vague to be used for evaluation if they
are not specific enough to determine the adequacy of performance before
the fact. Thus, a procedural rule such as "the anesthetic shall not begin until
the patient has been properly prepared for surgery" is imprecise, whereas
another such as "flammable anesthetic agents shall not be used" is specific.
When the rules are codified as written policies, imprecise rules usually
function simply to provide administrative hierarchies the opportunity to
assign blame to operators after accidents and to finesse the larger institu-
tional responsibility for creating the circumstances that lead to accidents
(see the report on the aircraft accident at Dryden, in Moshansky, 1992).
Significantly, the value of both the normative and standard practices meth-
ods of evaluating the problem-solving process of human operators is limited
to the most simple systems and generally fails as system size and complexity
A third approach is called the
neutral observer criterion
by De Keyser and
Woods (1990). The neutral observer criterion is an empirical approach that
compares practitioner behavior during the incident in question to the behav-
ior of similar practitioners at various points in the evolving incident. In
practice, the comparison is usually accomplished by using the judgment of
similar practitioners about how they would behave under similar circum-
stances. Neutral observers make judgments or interpretations about the
state of the world (in this domain, the patient and related monitors and
equipment), relevant possible future event sequences, and relevant courses
of action. The question is whether the path taken by the actual problem
solver is one that is plausible to the neutral observers. One key is to avoid
contamination by hindsight bias; knowledge about the later outcome may
alter the neutral observers' judgment about the propriety of earlier re-
sponses. The function of the neutral observer is to help define the envelope
of appropriate responses given the information available to the practitioner
at each point in the incident.
The writers' research, and that of others, is based on the development of
neutral observer criteria for actions in complex systems. This method in-
volves comparing actions that were taken by individuals to those of other
experts placed in the same situation. Note that this is a strong criterion for
comparison and it necessarily requires that the evaluators possess the same
sort of expertise and experience as was employed during the incident. It
does not rely on comparing practitioner behaviors with theory, rules, or
policies. It is particularly effective for situations where the real demands of
the system are poorly understood and where the pace of system activity is
high (i.e., in large, complex systems). The writers have used this technique in
examining human performance in incidents from several different sources in
anesthesia and in other domains. The technique is complex, as the descrip-
tions and discussions of the three exemplar incidents indicate, but the
complexity simply matches that of the domain and the human behaviors
being evaluated.
Did the
Commit Errors?
The three exemplar incidents in this chapter are not remarkable or unusual;
rather they reflect the normal, day-to-day operations that characterize busy,
urban tertiary care hospitals. In each incident, human performance is closely
tied to system performance and to eventual outcome, although the perfor-
mance of the practitioners is not the sole determinant of outcome. The
myocardial infarction following the events of Incident #1 may well have
happened irrespective of any actions taken by practitioners. That patient
was likely to have an infarction, and it is not possible to say whether the
anesthetist's actions caused the infarction. The incidents and the analysis of
human performance that they prompt (including the role of latent failures in
system transients) may make us change our notion of what constitutes a
human error.
Arguably, the performance in each exemplar incident is flawed. In retro-
spect, things can be identified that might have been done differently and that
would have forestalled or minimized the incident or its effect. In the myocar-
dial infarction incident, intravascular volume was misassessed, and treat-
ment for several simultaneous problems was poorly coordinated. In the
hypotension incident (#2), the device set-up by practitioners probably con-
tributed to the initial fault. The practitioners were also unable to diagnose
the fault until well after its effects had cascaded into a near crisis. In the
scheduling incident (#3), a practitioner violated policy. She chose one path
in order to meet certain demands, but simultaneously exposed the larger
system to a rare but important variety of failure. In some sense, each of the
exemplar incidents constitutes an example of human error. Note, however,
that each incident also demonstrates the complexity of the situations con-
fronting practitioners and the way in which practitioners adjust their behav-
ior to adapt to the unusual, difficult, and novel aspects of individual situa-
Especially in the hypotension incident (#2), the resiliency of human
performance in an evolving incident is demonstrated. The practitioners
were willing to abandon their efforts at diagnosis and shift to
mode of response in order to preserve the patient's life pending
resolution of the disturbance. The practitioner was also busy during the
myocardial infarction incident, although in this instance the focus was pri-
marily on producing better oxygenation of the blood and control of the
blood pressure and not on correcting the intravascular volume. These efforts
were significant and, in part, successful. In both Incidents #1 and #2, atten-
tion is drawn to the practitioner performance by the outcome.
In retrospect, some would describe aspects of these incidents as human
error. The high urine output with high blood glucose and prior administra-
tion of furosemideshouldhave prompted the consideration of low (rather
than high) intravascular volume. The infusion devices
have been set
up correctly, despite the complicated set of steps involved. The diagnosis of
have included a closer examination of the infusion
devices and their associated bags of fluid, despite the extremely poor device
feedback. Each of these conclusions, however, depends on knowledge of the
outcome; each conclusion suffers from hindsight bias. To say that something
have been obvious, when it manifestly was not, may reveal more
about our ignorance of the demands and activities of this complex world
than it does about the performance of its practitioners. It is possible to
generate an infinite list of shoulds for practitioners in anesthesiology and
other large systems, but these lists quickly become unwieldy and, in any
case, focus only on the most salient failures from the most recent disaster. It
is easy to slip into the "cycle of error" (Fig. 13.5), focusing on error out of
context, increasing the complexity of the larger system, exacerbating con-
flicts, and creating more opportunities for latent failures to accumulate and
come together in an accident.
The scheduling incident (#3) is somewhat different. In that incident, it is
clear how knowledge of the outcome biases the evaluations of practitioner
performance. As Abraham Lincoln said, "If the end brings me out all right
what is said against me won't amount to anything. If the end brings me out
wrong, ten angels swearing I was right will make no difference." Is there a
human error in Incident #3? If a trauma case had occurred in this interval
where all the resources had been committed to other cases, would her
decision then be considered an error? On the other hand, if she had delayed
the start of some other case in order to be prepared for a possible trauma
case that never happened and the delay contributed to some complication
for that patient, would her decision then be considered an error?
From this discussion, we are being forced to conclude that the human
error is a judgment made in hindsight. In a real sense, then, for scientists and
there is no such thing as human error
Hollnagel, 1993).
Human error does not comprise a distinct category of human performance.
As the incidents suggest, human performance is not simply either adequate
or inadequate. Neither is it either faulty or fault-free. Rather, human perfor-
mance is as complex and varied as the domain in which it is exercised.
Credible evaluations of human performance must be able to account for all
of the complexity that confronts the practitioner. This is precisely what most
evaluations of human performance do not do: They simplify the situations
and demands confronting practitioners until it is obvious that the practitio-
ners have erred. By stripping away the complexities and contradictions
inherent in operating these large systems, the evaluators eliminate the
richness of detail that might help to show how the activities of the practitio-
ners were locally rational and miss the bottlenecks and dilemmas that
challenge practitioner expertise and skill. The term human error should not
represent the concluding point but rather the starting point for studies of
accident evolution in large systems.
The schema of knowledge factors, attentional dynamics, and strategic
factors provide one means of categorizing the activities of teams of practitio-
ners." The model of large system failure arising from the concatenation of
multiple small latent failures provides an explanation for the mysteriously
unique appearance of failures. That model also explains the limited success
achieved by the pursuit of first causes in the cycle of error. It also suggests
that the human practitioner's role in large systems may be in part to un-
couple elements of the system in order to minimize the propagation of the
latent failures resident in the system (Perrow, 1984).
Together, the exemplar incidents and their analyses imply that many of
the changes occurring in medical practice may make the system more brittle
and increase the apparent contribution of human error. In response to
incidents, organizations generate more rules, regulations, policies, and pro-
cedures that make it more likely that medical practitioners will be found to
have erred by postincident analyses (Fig. 13.5). Emphasis on cutting costs
and increasing efficiency generates more pressure on practitioners, making
scenarios like that of the scheduling incident more likely. Increased use of
technology such as the computer-based infusion devices in the hypotension
incident (#2) raises the complexity of incidents and creates new modes of
failure. Even the burgeoning volume of medical knowledge plays a role,
making the likelihood of the sort of inert knowledge problems of the
myocardial infarction incident more probable (Feltovich et al., 1989). In the
face of these pressures, a quality management system that steadfastly main-
tains that human error is the root cause of system failures can be relied on to
generate a huge volume of error statistics that, in turn, become part of the
cycle of error and its consequences.
"The practitioners need not be human; the same schema may be used for evaluating the
performance of machine "expert systems" and the performance of teams of human and machine
cognitive agents.
If human performance is critical to system performance, then it seems
reasonable to try to enhance it. One method of improving human perfor-
mance is retraining. Unfortunately, most retraining is predicated on the
presence of a human error problem, that is, that flawed human performance
is the root cause of system failures and that eliminating this failure mode is
the key to success. Under this assumption, many training programs consist
merely of routinization of tasks according to a rote method. This approach is
sometimes called blame and train, because it begins with the concept that
human error is the source of the problem and that this error arises from
capriciousness or inattentiveness by practitioners. This was exactly what
happened following the Three Mile Island accident in 1979. The regulatory
agencies and organizations responded in part with an emphasis in training
on rote performance of compiled procedures, and the result was that opera-
tional personnel confronted a variety of dilemmas about whether to depart
from the standard procedures in more complicated incidents (Woods et al.,
There are several methods in use in the aviation and anesthesia domains
that represent contrasting approaches to training. Cockpit resource man-
agement (CRM) is a tool used by several major air carriers in an effort to
mprove crew performance (Wiener, Kanki, & Helmreich, 1993). CRM
acknowledges that air crews are a resource for solving problems and at-
tempts to give the crews more experience in working together in crisis
situations where coordination is critical. Unlike blame and train methods
that seek to regiment human performance to eliminate human error, CRM
implicitly views human performance as the critical resource in dealing with
novel, threatening situations and focuses on developing in pilots and engi-
neers the ability to work together as a coordinated team. In anesthesia,
Gaba's group at Stanford developed a similar tool called Crisis Resource
Management (CRM) that provides anesthetists with opportunities to see
themselves act under the pressure of simulated disasters (Gaba & DeAnda,
1989). Gaba's group uses material from the aviation CRM as well. Again,
the implicit view of this training method is that the human practitioner is a
resource and the only likely source of system resilience in the event of an
incident. The anesthesia CRM concentrates on infrequently experienced
but quite realistic scenarios (e.g., a complete power outage) as a test bed for
mproving human performance. Both CRMs are qualitatively different from
the majority of training approaches generally in use. Both make extensive
use of elaborate simulators and large bodies of domain knowledge and can
be quite expensive.
All of the large systems with which we are concerned (anesthesia, nuclear
power operations, aviation) are intensely technological, so much so that
they do not exist apart from their technology. During the past decade, each
of these domains has seen the introduction of automation, the purpose of
which is to eliminate human activity as the source of errors. The record of
these systems is mixed and controversial (Woods et al., 1994).
Much technological innovation is supposed to reduce human error by
reducing human workload. The introduction of microprocessor-controlled
infusion pumps, for example, can eliminate the cumbersome process of
adjusting a manual valve to regulate drips. However, these same devices
create other demands, including set-up, operation, and fault diagnosis as
seen in the hypotension incident. Similar equipment in the cockpit and the
operating room are actually examples of
clumsy automation
(Cook et al.,
1990; Wiener, 1989), where the workload reduction is limited to the times
when the operator is not busy at any rate (e.g., mid-flight), and the cost is a
substantial increase in workload at peak workload times (e.g., takeoff and
landing). Such systems are poor amplifiers of human performance and are
likely to degrade performance at critical times." Clumsy automation also
includes technologies that provide great increases in precision but demand
equally precise operation, such as the newer generation of drug infusion
pumps. One such device has a library of drug concentration data and is able,
given suitable input parameters, to compute and deliver precisely metered
doses of potent drugs based on patient weight. The set-up procedure for this
device is more complicated and time consuming than its predecessors and
increases the potential for large (i.e., order of magnitude) errors in dosing.
may also encounter black box systems whose behavior is
extraordinarily difficult to understand even when the function performed is
relatively simple (Cook, Potter, Woods, & McDonald 1991).
Another use of technology is to eliminate human decision making as a
source of error by eliminating the decision making entirely. This trend is
most advanced in the commercial aircraft cockpit, but it has also been
demonstrated in the operating room and the nuclear plant control center.
One effect of attempting to automate decision making can be to increase the
intensity of the responsibility-authority double bind. In the larger context,
practitioners faced with such devices confront a double problem: Not only
do they have to understand the situation confronting them, but they must
also understand how the machine sees that situation and be able to evaluate
"This may be one reason practitioners are sometimes reluctant to embrace such technolo-
the machine's proposed responses to the situation (Roth et al., 1987; Sarter
& Woods, in press; Woods et al., 1994).
Eliminating Human Error Versus Aiding Human
Clearly, those strategies that derive from a desire to minimize human error
are different from those that seek to aid human performance. Rules, regula-
tions, sanctions, policies, and procedures are largely predicated on the belief
that human error is at the heart of large system failures and that a combina-
tion of restrictions and punishments will transform human behavior from
error to an error-free state. The same basis exists for some training and
technology programs, for example, blame and train and automated decision
systems, whereas others (notably CRM) regard human performance as the
primary means for dealing with system transients and look for ways to
produce more effective human performance. The distinction is an important
one and not simply a matter of degree; the choice of path depends critically
on the validity of the whole notion of human error.
Human operator performance in large systems and the failures of these
systems are closely linked. The demands that large, complex systems opera-
tions place on human performance are mostly cognitive. The difference
between expert and inexpert human performance depends on the timely
and appropriate action that in turn is shaped by knowledge factors,
attentional dynamics, and strategic factors. A brief examination of a few
incidents occurring in anesthesia practice has demonstrated that human
performance is complex in proportion to the complexity of the domain itself.
Analyses of the human role, especially those that take place after an incident
or accident, must provide a satisfactory account of that complexity and its
mpact on human decision making and activity. The schema of knowledge
factors, attentional dynamics, and strategic factors can provide a framework
for laying out the issues confronting practitioners at the sharp end.
There are at least two different ways of interpreting human performance
in complex systems. The conventional way views human performance as the
source of errors that can be eliminated by restricting the range of human
activity or eliminating the performer from the system. According to this
human error
is seen as a distinct category that can be counted and
This chapter has presented a second approach, one that views human
performance as the means for resolving the uncertainties, conflicts, and
competing demands inherent in large, complex systems (Hollnagel, 1993).
This view acknowledges the presence of both blunt and sharp ends of the
system. The blunt end, including regulatory bodies, administrative entities,
economic policies, and technology development organizations, can affect
sharp-end practitioners by creating and amplifying conflicts and by deter-
mining the resources available for resolving those conflicts. The analyses
guided by this approach explicitly avoid the term
human error
because it
obscures more than it reveals.
Human error is not a distinct category of human performance. After the
outcome is clear, any attribution of error is a social and psychological
judgment process, not a narrow, purely technical or objective analysis.
Outcome knowledge biases retrospective evaluations. Different judges with
different background knowledge of the events and context or with different
goals will judge the performance of human practitioners differently. Recog-
nizing the limits of the label
human error
can lead us in new, more fruitful
directions for improving the performance of complex systems (Woods et al.,
So how should we view a large, complex system failure? If a bad outcome
is seen as yet another incident containing one or more human errors by some
practitioners, that is, we adopt the conventional view, what shall we do then?
The options are few. We can try to train people to remediate the apparent
deficiencies in their behavior. We can try to remove the culprits from the
scene or, at least, prevent these sorts of defective people from becoming
We can try to police practitioner activities more closely.
This chapter suggests quite a different approach. It proposes that system
failures are a form of information about the system in which people are
embedded. They do not point to a single independent (and human) compo-
nent (a culprit) as the source of failure. Instead, system failures indicate the
need for analysis of the decisions and actions of individuals and groups
embedded in the larger system that provides resources and constraints. To
study human performance and system failure requires studying the function
of the system in which practitioners are embedded. Failures tell us about
situations where knowledge is not brought to bear effectively, where the
attentional demands are extreme, where the
bind is created. Knowl-
edge of these systemic features allows us to see how human behavior is
shaped and to examine alternatives for shaping it differently.
In this view, the behavior that people, in hindsight, call "human error" is
the end result of a large number of factors coming to bear at the sharp end of
practice. Social and psychological processes of causal attribution lead us to
label some practitioner actions as "human error" and to regard other actions
as acceptable performance. Hindsight bias leads us to see only those forks in
the road that practitioners decided to take-we see "the view from one side
of a fork in the road, looking back" (Lubar, 1993, p. 1168). This view is
fundamentally flawed because it does not reflect the situation confronting
the practitioners at the scene. The challenge we face as evaluators of human
performance is to reconstruct what the view was like or would have been like
had we stood on the same road.
The few examples in this chapter give the flavor of what operating at the
sharp end really demands of practitioners. It is not surprising that human
operators occasionally should be unable to extract good outcomes from the
conflicted and contradictory circumstances in which they work. The surprise
is that they are able to produce good outcomes as often as they do.
This preparation of this chapter was supported in part by grants from the
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... Our goals are to expose various design "errors" in human-computer systems that create latent failures, show how devices with these characteristics shape practitioner cognition and behavior, and how these characteristics can create new possibilities for error and new paths to disaster. In addition, we will examine data on how practitioners cope with the complexities introduced by the clumsy use of technological possibilities and how this adaptation process can obscure the role of design and cognitive system factors in incident evolution (Woods et al., 1992;Cook and Woods, 1994). This information should help developers detect, anticipate, and recover from designer errors in the development of computerized devices. ...
... Failures very often can be traced back to strategic dilemmas and tradeoffs that arise from multiple interacting and sometimes conflicting goals. Practitioners by the very nature of their role at the sharp end of systems must implicitly or explicitly resolve these conflicts and dilemmas as they are expressed in particular situations (Cook and Woods, 1994). ...
... The next three sections from Cook and Woods (1994) explore in more detail how various knowledge factors, attcntional dynamics, and strategic factors govern the expression of expertise and error in distributed cognitive systems. To accomplish this, we will introduce each section with an actual incident that we have investigated ourselves taken from the field of anesthesiology. ...
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original version of BEhind Human Error. Second edition 2010 from Ashgate
... A fixation occurs when a situation assessment or course of action has failed to revise more evidence about problems in attentional dynamics. In this dimension, attention is a critical factor that moderates situational awareness; and training in mental skills is needed to enhance attention management and reduce the impact of stress (63,67). For the "call for help if needed" dimension, desirable behaviors were concentrated on seeking help and consultation from an experienced colleague or experts outside the team. ...
... Consistent with our findings, Parush et al. (2011) indicated that the implicit and explicit coordination of information exchange in the form of providing situation-related information without request and obtaining the required information about the situation is in the team adaptability direction (58). Fixation error has been avoided by actively reassessing the situation (63). ...
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Introduction: Situational awareness (SA), as a nontechnical human factor, is critical to the success of a trauma team. This study aimed to identify representatives of behaviors supporting (desirable) and diminishing (undesirable) SA for trauma teams while performing the initial assessment of multi-trauma patients. Methods: This Nominal Group Technique Study was conducted on twenty attending physicians from various specialties affiliated with Tehran University of Medical Sciences, who were invited to a nominal group technique meeting in 2020. Participants were asked to write down their proposed behaviors in silence. Subsequently, each participant shared their list with the group in a round-robin format, and clarifications were made through discussion. After categorizing the ideas, we asked participants to rate each behavior's importance on a five-point Likert scale. The consensus was defined as ≥70% agreement on a rating of 4 and 5. Results: The final SA behaviors for the trauma team consisted of 29 (22 desirable and 7 undesirable) behaviors arranged in seven dimensions: resource allocation, anticipate and plan, avoid fixation errors, call for help if needed, prioritize attention, reassess patient, and shared mental model. The most important desirable and undesirable behaviors were identified in resource allocation (n=8) and avoid fixation errors (n=7) dimensions, respectively. Resource allocation behaviors consist of 'checking necessary equipment', 'allocating an alternative person(s) to do the required task if needed', 'assigning tasks to the right person(s)', and 'Addressing each team member with a requested task'. Avoid fixation errors behaviors were 'insisting on performing the procedure', 'making decisions without considering all available information', and 'emphasizing others' expertise in the diagnostic process'. Conclusion: The proposed team SA behaviors may be used in assessing the trauma team performance and training program to promote trauma team SA.
... Indeed, the idea of complex organisations really working in a linear manner is also mostly abandoned in the safety literature on complex sociotechnical systems, but that does not prevent the sharp end/blunt end metaphor from still being a central reference[15,[21][22][23][24]. ...
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We discuss what it is with representations of safety that makes them so powerful, and what is at stake when representations travel across contexts and scales. The discussion uses the sharp end/blunt end metaphor as a central case.
... 10 Reason argued that when these latent failures, lying dormant, aligned, catastrophic errors could follow and our knee-jerk reactions to these catastrophes could result on us focusing on the individuals at the 'sharp end' of the system. 11 However, currently, insight into patient safety incident management still identifies that blame of individuals is commonplace in the response to errors despite the movement towards 'systems thinking'. 12 13 Systems thinking, in its simplest, is appreciating both the explicit and tacit processes that surround a system of work. ...
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Ensuring organisations learn from patient safety incidents is a key aim for healthcare organisations. The role that human factors and systems thinking can have to enable organisations learn from incidents is well acknowledged. A systems approach can help organisations focus less on individual fallibility and more on setting up resilient and safe systems. Investigation of incidents has previously been rooted in reductionist methodologies, for example, seeking to find the ‘root cause’ to individual incidents. While healthcare has embraced, in some contexts, the option for system-based methodologies—for example, SEIPS and Accimaps—these methodologies and frameworks still operate from a single incident perspective. It has long been acknowledged that healthcare organisations should focus on near misses and low harms with the same emphasis as incidents resulting in high harm. However, logistically, investigating all incidents in the same way is difficult. This paper puts forward an argument for themed reviews of patient safety incidents and provides an illustrative template for theming incidents using a human factors classification tool. This allows groups of incidents relating to the same portfolio, for example, medication errors, falls, pressure ulcer, diagnostic error, to be analysed at the same time and result in recommendations based on a larger sample size of incidents and based on a systems approach. This paper will present extracts of the themed review template trialled and argues that thematic reviews, in this context, allowed for a better understanding of the system of safety around the mismanagement of the deteriorating patient.
... Experts believe that hhhealthcare quality and safety must be investigated within the framework of systems and con-textual factors in which errors and adverse events occur. [1][2][3][4][5][6][7][8] Vincent and colleagues describe several factors that influence clinical practice: organizational factors such as safety climate and morale, work environment factors such as staffing levels and managerial support, team factors such as teamwork and supervision, and staff factors such as overconfidence and being overly self assured. [8] Healthcare provider attitudes about these and related factors are one component of an organization's safety culture. ...
Objective: Management practice commonly assumes that the value of a work-goal dictates the nature of motivation processes. We investigate instead how individuals invest resources from the perspective of their own value system. Drawing from Conservation of Resources theory, we explore the valuation process by testing a reciprocal model between work-goal attainment, goal commitment and personal resources, including self-efficacy, optimism, and subjective well-being. Method: Data were collected in a two-wave longitudinal study among sales professionals (n= 793) from France (F), Pakistan (P), and the USA (U). Results: Multi-group cross-lagged path analysis confirmed the reciprocal model across all three countries. Time 1 resources and goal commitment predicted work goal attainment (F= .24; P= .37; U= .39) and (F= .31; P= .40; U= .36) respectively. T1 level of goal attainment also fuelled T2 resources and goal commitment (F=.30; P= .29; U=.34) and (F= .33; P=.32; U= .29). Conclusions: Our reciprocal findings suggest a revised approach on the nature of targets and goals. They indicate an alternative to linear path modelling, as the role of goal commitment is not necessarily that of an intermediary stage linking antecedent resources to attainment purposes. Furthermore, cultural values play a differentiating role in the goal attainment process.
... Despite efforts to employ automation, the process is still heavily dependent on human operators (pharmacists, nurses, physicians, etc.) [3], and technologies that aim to increase safety and decrease workload such as wireless smart infusion pump, suffer from usability problems [4] and delays in updating their drug libraries [5], that might result in serious harm. This environment in which the task is carried out is complex, affording limited capacity and presenting competing priorities that are always changing and not always agreed on [6]. Improving the process requires its being studied and designed as a complex socio-technical system, where human-systems integration enhances performance [7]. ...
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This paper aims to highlight how to reduce medication errors through the implementation of human factors science to the design features of medication containers. Despite efforts to employ automation for increased safety and decreased workload, medication administration in hospital wards is still heavily dependent on human operators (pharmacists, nurses, physicians, etc.). Improving this multi-step process requires its being studied and designed as an interface in a complex socio-technical system. Human factors engineering, also known as ergonomics, involves designing socio-technical systems to improve overall system performance, and reduces the risk of system, and in particular, operator, failures. The incorporation of human factors principles into the design of the work environment and tools that are in use during medication administration could improve this process. During periods of high workload, the cognitive effort necessary to work through a very demanding process may overwhelm even expert operators. In such conditions, the entire system should facilitate the human operator’s high level of performance. Regarding medications, clinicians should be provided with as many perceptual cues as possible to facilitate medication identification. Neglecting the shape of the container as one of the features that differentiates between classes of medications is a lost opportunity to use a helpful characteristic, and medication administration failures that happen in the absence of such intentional design arise from “designer error” rather than “user error”. Guidelines that define a container’s shape for each class of medication would compel pharmaceutical manufacturers to be compatible and would eliminate the confusion that arises when a hospital changes the supplier of a given medication.
... Unexpected incidents are common in critical care medicine (Cook et al 1991, Cook and Woods 1994, Gaba 1994. Anesthesiologists face them during 20 percent of all anesthetics. ...
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This chapter seeks to offer some explanation for the ubiquity of different types of visual representations in safety science. In particular, the chapter focuses on what these tell us about the thinking of safety researchers and practitioners, as well as how diagrams and other visual material influence their use of safety methods and tools.
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Safety visualisations and their influences on safety concepts are presented. Visualisations like safety posters show a clear message of fear and guilt. This changes after World War II, due to a more tolerant atmosphere. Latent, organisational factors as decisive elements of accident processes appear in visualisations. An example shows a method to follow accident scenarios in real time.
Conference Paper
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Il presente contributo, seguito diretto del “Paper 9 - Esperienze e formazione docente di innovazione didattica durante l’emergenza Covid19 - Un caso di studio reale con utilizzo di piattaforme di e-learning” - Convegno Didamatica2021 – “Artificial Intelligence for Education” organizzato da AICA in collaborazione con il CNR - 7-8 ottobre 2021 CNR Palermo - ISBN 9788898091621, affronta i temi dell’innovazione didattica e della formazione docenti, legati alla didattica innovativa. Nello specifico vengono presentata nuove esperienza di didattica legata alla realtà virtuale, la realtà aumentatae l’Internet of Things. Viene inoltre fatto un confronto con le metodologie e tecnologie utilizzate e gli applicativi utilizzati durante l’esperienza. Queste esperienze, come detto in precedenza, sono il proseguimento del percorso iniziato nel precedente anno scolastico. Link:
Technical Report
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This report describes research conducted during 1989 and 1990 on the cognitive characteristics of a corpus of anesthesia critical incidents. The incidents were collected by monitoring and transcribing the regular quality assurance conferences in a large, university anesthesiology department. The 57 reports of incidents were analyzed by constructing protocols which traced the flow of attention and the knowledge activation sequence of the participants. Characteristics of the resulting protocols were used to divide the collection into five categories: acute incidents, going sour incidents, inevitable outcome incidents, airway incidents, and non-incident incidents. Of these, the acute and going sour categories represent distinct forms of incident evolution. The implications of this distinction are discussed in the report. Nearly all of the incidents involve human cognitive performance features. Cognition clearly plays a role in avoiding incidents but also in aborting and recovering from incidents in progress. Moreover, it is clear that subtle variations in cognitive function may playa crucial role in anesthetic disasters, of which incidents are taken to be prototypes. Review of the corpus reveals the different cognitive functions involved in anesthesia and anesthesia incidents. These cover a wide range including classic aspects of cognition, for example the direction of attention, and complex and poorly understood aspects such as situation awareness. The cognitive features include dealing with competing goals, dealing with competing indicators, the limitations of imperfect models, knowledge activation failures, the role of learned procedures and assumptions in reducing cognitive workload, failure to integrate multiple themes, organizational factors, and planning. These presence of these different cognitive features and cognitive failures in a single discipline is significant because it enhances and supports separate findings from other domains (e.g. nuclear power plant operation, commercial aviation) and also because it provides strong support for the contention that operators acting in these semantically complex, time pressured, high consequence domains face common problems and adopt similar strategies for dealing with them. The report demonstrates the way in which cognitive analysis of incidents can be accomplished in anesthesia and in other domains and suggests a system for categorizing the results obtained. It also raises questions about the adequacy of evaluations of risk and safety that do not explicitly account for the cognitive aspects of incidents and their evolution. In order to make real progress on safety in domains that depend critically on human operators it is necessary to examine and assess human cognitive performance, a process which requires large amounts of data and careful reconstruction. Such cognitive analysis is difficult. It requires substantial experience, skill, and effort and depends on acquiring and sifting through large quantities of data. This should not be suprising, since the domain itself is one characterized by experience, skill, effort, and large quantities of data. The challenge for us and for other researchers is to perform more such analyses and extend and refine the techniques described here and to link the analyses to those from other domains.
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Many critical real world human problem solving situations take place in dynamic event-drivers environments, where the evidence comes over time and situations can change rapidly. In these situations people must amass and integrate, uncertain, incomplete and changing evidence. A major source of human error in dynamic domains seems to be a failure to revise situation assessment as new evidence comes in. This paper will be concerned with the identification of the main descriptive patterns of fixation errors and with how to build new sytems to reduce this type of error. It will also begin the process of building a theory of fixation errors.
One result of recent research on human error and disaster is that the design of the human-machine system, defined broadly, modulates the potential for erroneous action. Clumsy space use of technological powers can create additional mental burdens or other constraints on human performance that can increase the chances of erroneous actions by people especially in high workload, high tempo operations. This paper describes studies of a computer based automated device that combine critical incident analysis, bench test evaluations of the device, and field observation in order to understand physician-device interaction in the context of heart surgery. The results link, for the same device, user group and context, three findings. First, the device exhibits classic human-computer interaction flaws such as lack of feedback on device state and behavior. Second, these HCI flaws actually do increase the potential for erroneous actions and increase the potential for erroneous assessments of device state and behavior. The potential for erroneous state assessment is especially troublesome because it impairs the user’s ability to detect and recover from misassemblies, misoperations and device failures. Third, these data plus critical incident studies directly implicate the increased potential for erroneous setup and the decreased ability to detect errors as one kind of important contributor to actual incidents. The increased potential for error that emanates from poor human-computer interaction is one type of latent failure that can be activated and progress towards disaster given the presence of other potentiating factors in Reason’s model of the anatomy of disasters.
Patrolling the restricted waters of the Persian Gulf was a trying activity for most U.S. warships, designed, armed, and trained as they were for far-ranging “blue water” operations. This was particularly true for the officers and crew of the USS Vincennes. One of the first of the Ticonderoga-class “Aegis” cruisers, the Vincennes is a fast, lightly armored ship—a cruiser built on a large destroyer hull—specially optimized for fleet air defense. Although armed with various surface-to-surface guns and a variety of systems for close-in air defense, her real “main battery” consisted of the Standard SM-2 anti-aircraft missiles stored deep in her magazines.
An efficient strategy for fault diagnosis relies upon specific symptoms to activate only a small portion of the available diagnostic knowledge. However, expert systems and humans using this strategy often commit characteristic errors. These errors occur because significantly different faults may manifest similar symptoms. If these symptoms are confused, the wrong portion of diagnostic knowledge can be activated. This paper discusses such errors and describes a strategy for reducing their incidence based upon an investigation of expertise in the Galen expert system.