Science Overlay Maps: A New Tool for Research Policy and Library Management

Abstract

We present a novel approach to visually locate bodies of research within the sciences, both at each moment of time and dynamically. This article describes how this approach fits with other efforts to locally and globally map scientific outputs. We then show how these science overlay maps help benchmark, explore collaborations, and track temporal changes, using examples of universities, corporations, funding agencies, and research topics. We address conditions of application, with their advantages, downsides and limitations. Overlay maps especially help investigate the increasing number of scientific developments and organisations that do not fit within traditional disciplinary categories. We make these tools accessible to help researchers explore the ongoing socio-cognitive transformation of science and technology systems. Comment: 40 pages, 6 Figures

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SWEPS
SPRU Electronic Working Papers Series
Paper No. 179
Science overlay maps:
a new tool for research policy
Ismael Rafols
SPRU, University of Sussex and
TPAC, Georgia Institute of Technology
Alan L. Porter
TPAC, Georgia Institute of Technology
Loet Leydesdorff
ASCoR, University of Amsterdam
December 2009
SPRU, The Freeman Centre
University of Sussex
Falmer, Brighton, BN1 9QE
w.e.steinmueller@sussex.ac.uk
http://www.sussex.ac.uk/spru

Science overlay maps: a new tool for research policy
Ismael Rafols
1
, Alan L. Porter
2
and Loet Leydesdorff
3
1
SPRU –Science and Technology Policy Research, University of Sussex,
Brighton, BN1 9QE, England; i.rafols@sussex.ac.uk ;
2
Technology Policy and Assessment Center, Georgia Institute of Technology,
Atlanta, GA 30332, USA; alan.porter@isye.gatech.edu; and Search Technology, Inc.
3
Amsterdam School of Communication Research (ASCoR), University of
Amsterdam,
Kloveniersburg 48, 1012 CX Amsterdam, The Netherlands; loet@leydesdorff.net
Abstract
We present a novel approach to visually locate bodies of research within the
sciences, both at each moment of time and dynamically. This article describes how
this approach fits with other efforts to locally and globally map scientific outputs. We
then show how these science overlay maps help benchmark, explore collaborations,
and track temporal changes, using examples of universities, corporations, funding
agencies, and research topics. We address conditions of application, with their
advantages, downsides and limitations. Overlay maps especially help investigate the
increasing number of scientific developments and organisations that do not fit within
traditional disciplinary categories. We make these tools accessible to help
researchers explore the ongoing socio-cognitive transformation of science and
technology systems.
Keywords: science, map, overlay, classification, interdisciplinary, research,
evaluation.

1. Introduction
Most science and technology institutions have undergone or are undergoing major reforms
in their organisation and in their activities in order to respond to changing intellectual
environments and increasing societal demands for relevance. As a result, the traditional
structures and practices of science, built around disciplines, are being by-passed by
organisation in various ways in order to pursue new types of differentiation that react to
diverse pressures (such as service to industry needs, translation to policy goals, openness
to public scrutiny, etcetera). However, no clear alternative socio-cognitive structure has yet
replaced the “old” disciplinary classification. In this fluid context, in which social structure
often no longer matches with the dominant cognitive classification in terms of disciplines, it
has become increasingly necessary for institutions to understand and make strategic
choices about their positions and directions in moving cognitive spaces. “The ship has to be
reconstructed while a storm is raging at sea.” (Neurath, 1932/33) The overlay map of
science we present here is a technique that intends to be helpful in responding to these
needs elaborating on recently developed global maps of science (Leydesdorff & Rafols,
2009).
Although one would expect global maps of science to be highly dependent on the
classification of publications, the clustering algorithms, and visualisation techniques used,
recent studies comparing maps created using very different methods revealed that, at a
coarse level, these maps are surprisingly robust (Klavans & Boyack, 2009; Rafols &
Leydesdorff, 2009). This stability allows to ‘overlay’ publications or references produced by a
specific organisation or research field against the background of a stable representation of
global science and to produce comparisons that are visually attractive, very readable, and
potentially useful for science policy-making or research and library management. In this
study, we present one such overlay technique and introduce its possible usages by
practitioners by providing some demonstrations. For example, one can assess a portfolio at

the global level or animate a diffusion pattern of a new field of research. We illustrate the
former application with examples from universities, industries and funding agencies, and the
latter for an emergent research topic (carbon nanotubes). In appendices we provide the
technical information for making these overlays using software available in the public
domain.
Our first objective is to introduce the method for making and/or utilising the global maps to
prospective users in the wider science policy and research management communities who
are not able to follow the developments in scientometrics in detail. Since the paper
addresses a wide audience, we shall not discuss technical bibliometric issues, but provide
references to further literature. Secondly, we reflect on issues about the validity and
reliability of these maps. Thirdly, this study explores the qualitative conditions of application
of the maps, proposing examples of meaningful usage and flagging out potential
misreadings and misunderstandings.
As classifications, maps can become embedded into working practices and turn into habit, or
be taken for granted away from public debate, yet still shaping policy or management
decisions that may benefit some groups at the expense of others (Bowker & Star, 2000, pp.
319-320). In our opinion, scientometric tools remain error-prone representations and fair use
can only be defined reflexively. Maps, however, allow for more interpretative flexibility than
rankings. By specifying the basis, limits, opportunities and pitfalls of the global and overlay
maps of science we try to avoid the widespread problems that have beset the policy and
management (mis-)use of bibliometric indicators such as the impact factor (Martin, 1997;
Gläser & Laudel, 2007). By specifying some of the possible sources of error, we aim to set
the conditions so that this novel tool remains open to critical scrutiny and can be used in an
appropriate and responsible manner (Rip, 1997, p. 9).

2. The dissonance between the epistemic and social structures of science
The traditional representation of science was derived from the so-called ‘tree of knowledge’
according to which metaphor, knowledge is split into branches, then into major disciplines
and further differentiated into subdisciplines and specialties. The modern universities mainly
organised their social structure along this model (Lenoir, 1997), with a strong belief that
specialisation was key for successful scientific endeavour (Weber, 1919). However, many (if
not most) scientific activities no longer align with disciplinary boundaries (Whitley, 1984
(2000); Klein, 2000; Stehr & Weingart, 2000).
1
As Lenoir (1997, p. 53) formulated:
Scientists at the research front do not perceive their goal as expanding a discipline.
Indeed most novel research, particularly in contemporary science, is not confined
within the scope of a single discipline, but draws upon work of several disciplines. If
asked, most scientists would say that they work on problems. Almost no one thinks of
her- or himself as working on a discipline.
The changing social contract of science, progressively enacted in the last 20 years, has
brought a stronger focus on socio-economic relevance and accountability (Gibbons et al.
1994; Etzkowitz & Leydesdorff, 2000), which has exacerbated the dissonances between
epistemic and organisational structures. Descriptions of recent transformations emphasise
inter-, multi-, or transdisciplinary research as a key characteristic of the new forms of
knowledge production (reviewed by Hessels & Van Lente, 2008).
These ongoing changes pose challenges to the conduct and institutional management of
science and higher education. New ‘disciplines’ that emerged in the last decades, such as
1
The ‘tree of knowledge’ (e.g., Maturana and Varela, 1984) has strong similarities with the ‘tree of life’
developed by biology (via the subdicipline of systematics) to explain the diversity of species out of a
common origin. Interestingly, the ‘tree of life’ has also been increasingly challenged by evidence of
massive horizontal gene transfer among prokaryotes (Bapteste et al. 2009).

computer or cognitive sciences do not fit neatly into the tree of knowledge. Demands for
socially relevant research have also led to the creation of mission-oriented institutes and
centres targeting societal problems, such as mental health or climate change, that spread
(and sometimes cross-fertilise) across disciplines. At the institutional level, however, one
cannot avoid the key question of the relative position of these emergent organisations and
fields in relation to ‘traditional’ disciplines when it comes to the evaluation. Can changes in
research areas be measured against a baseline (Leydesdorff et al., 1994; Studer & Chubin,
1982)? Are the new developments transient (Gibbons et al., 1994) or, perhaps, just relabeling
of “old wine” (Van den Daele et al., 1979; Weingart, 2000)? Such questions point to our
endeavour: can science overlay maps be a tool to explore the increasingly fluid and complex
dynamics of the sciences? Do they allow us to throw light upon the cognitive and
organisational dynamics, thereby facilitating research-related choices (e.g., funding,
organization)?
3. Approaches to mapping the sciences
Science maps are symbolic representations of scientific fields or organisations in which the
elements of the map are associated with topics or themes. Elements are positioned in the
map so that other elements with related or similar characteristics are located in their vicinity,
while those elements that are dissimilar are positioned at distant locations (Noyons, 2001, p.
84). The elements in the map can be authors, publications, institutes, scientific topics, or
instruments, etc. The purpose of the representation is to enable the user to explore relations
among the elements.
Science maps were developed in the 1970s (Small 1973; Small & Griffith, 1974; Small &
Sweeny, 1985; Small et al., 1985). They underwent a period of development and dispute
regarding their validity in the 1980s (Leydesdorff, 1987; Hicks, 1987; Tijssen et al., 1987),
and a slow process of uptake in policy during the 1990s, that fell below the expectations

created (Noyons, 2001, p. 83). The further development of network analysis during the
1990s made new and more user-friendly visualisation interfaces available. Enhanced
availability of data has spread the use and development of science maps during the last
decade beyond the scientometrics community, in particular with important contributions by
computer scientists specialised in the visualisation of information (Börner et al. 2003), as
illustrated by the educative and museological exhibition, Places and Spaces
(http://www.scimaps.org/ ).
Most science maps use data from bibliographic databases, such as PubMed, Thomson
Reuters’ Web of Science or Elsevier’s Scopus, but they can also be created using other data
sources (e.g., course pre-requisite structures, Balaban & Klein, 2006). Maps are built on the
basis of a matrix of similarity measures computed from correlation functions among
information items present in different elements (e.g. co-occurrence of the same author in
various articles). The multidimensional matrices are projected onto two or three dimensions.
Details of these methods are provided by Leydesdorff (1987), Small (1999) and reviewed by
Noyons (2001, 2004) and Börner et al. (2003).
In principle, there are several advantages of using maps rather than relying just on numeric
indicators. Maps position units in a network instead of ranking them on a list. As in any data
visualisation technique, maps furthermore facilitate the reading of bibliometric information by
non-experts—with the downside that they also leave room for manipulating the interpretation
of data structures. Second, maps allow for the representation of diverse and large sets of
data in a succinct way. Third, precisely because they make it possible to combine different
types of data, maps also enable users to explore different views on a given issue. This
interpretive flexibility induces reflexive awareness about the phenomenon the user is
analysing and about the analytical value (and pitfalls) of these tools. Maps convey that
bibliometrics cannot provide definite, ‘closed’ answers to science policy questions (such as
“picking the winners”). Instead, maps remain more explicitly heuristic tools to explore and

potentially open up plural perspectives in order to inform decisions and evaluations
(Roessner, 2000; Stirling, 2008).
While the rhetoric of numbers behind indicators can easily be misunderstood as objectified
and normalized descriptions of a reality (the “top-10”, etc.), the heuristic, toy-like quality of
science maps is self-exemplifying. These considerations are important because ‘[T[here is a
lot of misunderstanding [by users] about the validity and utility of the maps’ (Noyons, 2004,
p. 238). This is compounded with a current lack of ethnographic or sociological validation of
the actual use of bibliometric tools (Woolgar, 1991; Rip, 1997; Gläser & Laudel, 2007).
The vast majority of science maps have aimed at portraying local developments in science,
using various units of analysis and similarity measures. To cite just a few techniques:
• co-citations of articles (e.g. research on collagen, Small, 1977);
• co-word analysis (Callon et al., 1986), e.g. translation of cancer research (Cambrosio
et al., 2007);
• co-classification of articles (e.g. neural network research, Noyons & Van Raan,
1998);
• co-citations of journals (e.g. artificial intelligence, Van den Besselaar & Leydesdorff,
1996);
• co-citation of authors (e.g. information and library sciences, White & McCain, 1998).
These local maps are very useful to understand the internal dynamics of a research field or
emergent discipline, but typically they cover only a small area of science. Local maps have
the advantage of being potentially accurate in their description of the relations within a field
studied, but the disadvantage is that the units of analyses and the positional co-ordinates
remain specific to each study. As a result, these maps cannot teach us how a new field or
institute relates to other scientific areas. Furthermore, comparison among different

developments is difficult because of the different methodological choices (thresholds and
aggregation levels) used in each map.
Shared units of representation and positional co-ordinates are needed for proper
comparisons between maps. In order to arrive at stable positional co-ordinates, a full
mapping of science is needed. In summary, two requirements can be formulated as
conditions for a global map of science: mapping of a full bibliographic database, and robust
classification of the sciences. Both requirements were computationally difficult until the last
decade and mired in controversy. The next section explains how these controversies are in
the process of being resolved and a consensus on the core structure of science is emerging.
4. Global maps of science: the emerging consensus
The vision that a comprehensive bibliographic database contained the structure of science
was already present in the seminal contributions of Price (1965). From the 1970’s, Henry
Small and colleagues at the Institute of Scientific Information (ISI) started efforts to achieve a
global map of science. In 1987, the ISI launched the first World Atlas of Science (Garfield,
1987) based on co-citation clustering algorithms. However, the methods used (single-linked
clustering) were seen as unstable and problematic (Leydesdorff, 1987). Given the many
choices that can be made in terms of units of analysis, measures of similarity/distance,
reduction of dimensions and visualisation techniques (Börner et al., 2003), most researchers
in the field (including ourselves) expected any global science representations to remain
heavily dependent on these methodological choices (Leydesdorff, 2006).
Against these expectations, recent results of a series of global maps suggest that the basic
structure of science is surprisingly robust. First, Klavans & Boyack (2009) reported a
remarkable degree of agreement in the core structure of twenty maps of science generated
by independent groups, in spite of different choices of unit of analysis, similarity measure,

classification (or clustering algorithms) or visualisation technique
2
.Then, Rafols & Leydesdorff
(2009) showed that similar global maps can be obtained using significantly ‘dissenting’ journal
classifications. These validations emphasize bibliometric, rather than expert assessment
(Rip, 1997, p. 15), but this seems suitable in considering global science mappings, given that
no experts are capable of making reliable judgement on the interrelations of all parts of
science (Boyack et al., 2005, p. 359; Moya-Anegón et al, 2007, p. 2172). The consensus is
more about the coarse structure of science than on final maps. The latter may show apparent
discrepancies due to different choices of representation. This is the case, for example, when
one compares Moya-Anegón et al. (2007) use of fully centric maps as opposed to Klavans &
Boyack’s (2008) fully circular ones.
Let us explore key features of the emerging consensus on the global structure, illustrating
with Figure 1. The first feature is that science is not a continuous body, but a fragmentary
structure composed of both solid clusters and empty spaces—in geographical metaphors, a
rugged landscape of high mountains, and deep valleys or faults rather than plains with rolling
hills. This quasi-modular structure (or “near decomposability” in terms of the underlying
(sub)systems) can be found at different levels. This multi-level cluster structure is related to
the power-law distributions in citations (Katz, 1999). Furthermore, these multi-level
discontinuities of science are consistent with qualitative descriptions (Dupré, 1993; Galison &
Stump, 1996; Abbot, 2001).
A first view of Figure 1 at the global level reveals a major biomedical research pole (to the
left in Figure 1), with molecular biology and biochemistry at its centre, and a major physical
sciences pole (to the right in Figure 1), including engineering, physics and material sciences.
2
More recently developed maps also show a high degree of agreement, in spite of using very
different methods such as hybrid text/citation clustering (Janssens et al. 2009) or click-stream by
users of journal websites (Bollen et al. 2009).

A third pole would be constituted by the social sciences and the humanities (at the bottom
left in Figure 1).
3
The second key feature is that the poles described above are arranged in a somewhat
circular shape (Klavans & Boyack, 2009)—rather than a uniform ring, more like an uneven
doughnut (a torus-like structure) that thickens and thins at different places of its perimeter.
This doughnut shape can best be seen in three-dimensional representations; it is not an
artefact produced by the reduction of dimensions or choice of algorithm used for the
visualisation. The torus-like structure of science is consistent with a pluralistic understanding
of the scientific enterprise (Knorr-Cettina, 1999; Whitley, 2000): in a circular geometry no
discipline can dominate by occupying the centre of science; and at the same time, each
discipline can be considered as at the centre of its own world.
The torus-like structure explains additionally how the great disciplinary divides are bridged.
Moving counter-clockwise from 3 o’clock to 10 o’clock in Figure 1 (see Figure 2 for more
details), the biomedical and the physical sciences poles are connected by one bridge that
reaches from material sciences to chemistry, and a parallel elongated bridge that stretches
from engineering and materials to the earth sciences (geosciences and environmental
technologies), then through biological systems (ecology and agriculture) to end in the
biomedical cluster. Moving from 10 o’clock to 6 o’clock, one can observe how the social
sciences are strongly connected to the biomedical cluster via a bridge made by cognitive
science and psychology, and a parallel bridge made by disciplines related to health services
(such as occupational health and health policy). Finally, moving from 6 o’clock to 3 o’clock,
3
The social sciences appear as a rather diffuse and small area in these science maps due to lower
citation rates. However, a recent study of social sciences on their own shows a cluster as large as the
natural sciences (Bollen et al, 2009).

we observe that the social sciences link back to the physical sciences via the weak
interactions in mathematical applications and between business and computer sciences.
4
The idea behind the emergent consensus is that the most important relations among
disciplines are robust—i.e. they can be elicited in the different maps even when their
representations differ in many details of the global science map due to other methodological
choices. However, one should not underestimate the differences among maps—particularly
since they can illuminate biases. In some cases, the disagreements are mainly visual like
those between geographic portrayals (e.g. in Mercator vs. Peters projections): although
there are different choices regarding the position and area size of Greenland, they all agree
that Greenland lies between North America and west Eurasia. However, in some other
cases, disagreements can be significant. For example, the position of mathematics (all math
subject categories) in the map remains open to debate. Since different strands of
mathematics are linked to different major fields (medicine, engineering, social sciences),
these may show as diverse entities in distant positions, rather than as a unitary corpus,
depending on classifications and/or clustering algorithms used.
It is important to recognize that the underlying relationships are multidimensional, so various
two (and three-) dimensional representations can result. For example, we depicted (in Figure
1) chemistry in the centre and geosciences on the periphery, but a 3D representation would
show that the opposite representation is also legitimate. Furthermore, due to reduction of
dimensions relative distances among categories need to be interpreted with caution, since
two categories may appear to be close without being similar. This is the case, for example,
for the categories “paper and wood materials science” and “paleontology” (at the top of our
basemap), or “dairy and animal science” and “dentistry” (top left). Categories that are only
4
Notice that the global map has circular symmetry, some branches develop in parallel over the
torus(e.g. geosciences and chemistry). As a result, the creation of a fully uni-dimensional wheel or
circle of science is very elegant and perhaps very useful, but it involves some distortions beyond the
consensus structure (see Klavans and Boyack, forthcoming or http://www.scival.com/).

weakly linked to a few other categories are particularly prone to generate this type of
positional ‘mirage.’ On the other hand, dimensional reduction also means that one can
expect ‘tunnels,’ whereby hidden dimensions closely connect apparently distant spaces in
the map. For example, “clinical medicine” and a small subset of engineering are connected
via a slim ‘tunnel’ made by “biomedical engineering and nuclear medicine.”
In summary, the consensus on the structure of science enables us to generate and warrant
a stable global template to use as a basemap. Several representations of this backbone are
possible, legitimate and helpful in bringing to the fore different lights and shadows. By
standardizing our mapping with a convenient choice (as shown in Figure 2), we can produce
comparisons that are potentially useful for researchers, science managers, or policy-makers.
For example, one can assess a portfolio at the global level or animate a diffusion pattern of a
new field of research.

Figure 1. The core structure of science. Cosine similarity of 18 macro-disciplines created
from factor analysis of ISI Subject Categories in 2007. The size of nodes is proportional to
number of citations produced.
Pajek
Cognitive Sci
Agricul Sci
Biomed Sci
Chemistry
Physics
Engineering
Env Sci & Tech
Matls Sci
Infectious
Diseases
Psychology
Social
Studies
Clinical
Medicine
Computer Sci
Business
& MGT
Ecol Sci
Econ Polit &
Geography
Health & Soc Issues
Geosciences
Figure 2. Global science map based on citing similarities among ISI Subject Categories
(2007).

5. Science overlay maps: a novel tool for research analysis
The local science maps are problematic for comparisons because they are not stable in the
units or positions of representation, as outlined in section 3. To overcome this, one can use
the units and the positions derived from a global map of science, but overlay on them the
data corresponding to the organisations or themes under study, as first shown by Boyack
(2009). In this section we introduce in detail a method of overlaying maps of science. This
method can be explored interactively in our webpage http://idr.gatech.edu/maps or
http://www.leydesdorff.net/overlaytoolkit.
5
A step-by-step guide on how to construct overlay
maps is provided in Appendix 1.
To construct the basemap, we use the subject categories (SCs) of the Web of Science to
which the ISI (Thomson Reuters) assigns journals based on journal-to-journal citation
patterns and editorial judgment. The SCs operationalise ‘bodies of specialized knowledge’
(or subdisciplines) to enable one to track the position of articles. The classification of articles
and journals into disciplinary categories is controversial and the accuracy of the ISI
classification is open to debate (Pudovkin & Garfield, 2002, at p. 1113n). Other
classifications and taxonomies are problematic as well (Rafols & Leydesdorff; 2009; NAS,
2009, p. 22). Bensman & Leydesdorff (2009) argued for using the classification of the Library
of Congress, but this extension would lead us beyond the scope of this study. However,
since the global maps have been shown to be relatively robust even when there is 50%
disagreement about classifications, we pragmatically choose the classification that has been
most widely used and is most easily accessible, despite its shortcomings (Rafols &
Leydesdorff, 2009; see Appendix 2).
5
A user-friendly toolkit using freeware Pajek is available at
http://www.leydesdorff.net/overlaytoolkit/sc2007.zip .

We follow the same method outlined in Leydesdorff & Rafols (2009), inspired by Moya-
Anegón et al. (2004). First, data were harvested from the CD-Rom version of the Journal
Citation Reports (JCR) of the Science Citation Index (SCI) and the Social Science Citations
Index (SSCI) of 2007, containing 221 Subject Categories (SCs). This data is used to
generate a matrix of citing SCs to cited SCs with a total of 60,947,519 instances of citations
between SCs. Salton’s cosine was used for normalization in the citing direction. Pajek is
used for the visualizations (http://pajek.imfm.si) and SPSS (v15) for the factor analysis.
Figure 2 shows the global map of science obtained using the 221 ISI SCs in 2007. Each of
the nodes in the map shows one SC, representing a subdiscipline. The lines indicate the
degree of similarity (with a threshold cutoff at a cosine similarity > 0.15) between two SCs,
with darker and thicker lines indicating stronger similarity. The relative position of the SCs is
determined by the pulls of the lines as a system of strings, depending on the extent of
similarity, based on the algorithm of Kamada and Kawai (1989). Although in this case we
used the ISI SCs, the same method was reproduced with other classification schemes
(Rafols & Leydesdorff, 2009).
The labels and colours in Figure 2 display 18 macro-disciplines (groupings of SCs) obtained
using factor analysis of this same matrix. The attribution of SCs to factors is listed in the file
221_SCs_2007_Citations&Similarities.xls provided in the supplementary materials
6
The
choice of 18 factors was set pragmatically since it was found that the 19
th
factor did not load
strongly to its own elements. Figure 1, which we used above to illustrate the discussion on
the degree of consensus, shows the core structure of science according to these18 macro-
categories.
The full map of science shown in Figure 2 provides the basemap over which we will explore
specific organisations or scientific themes using our ‘overlay’ technique. The method is
straightforward. First, the analyst retrieves a set of documents at the Web of Science. This
6
This matrix is also available at http://www.leydesdorff.net/overlaytoolkit/SC2007.xls .

set of documents is the body of research to be studied -- e.g., the publications of an
organisation, or the references (knowledge base) used in an emergent field, or the citations
(audience) to the publications of a successful laboratory. By assigning each document to a
category, the function Analyze provided in the Web of Science interface can be used to
generate a list of the number of documents present in each SC. Uploading this list, the
visualization freeware Pajek produces a map of science in which the size of a node (SC) is
proportional to the number of documents in that category. Full details of the procedure to
generate this vector are provided in Appendix 1.
Figure 3 illustrates the use of science overlay maps by comparing the profiles of three
universities with distinct strengths: the University of Amsterdam, the Georgia Institute of
Technology, and the London School of Economics (LSE). For each of them, the publications
from 2000 to 2009 were harvested and classified into SCs in the Web of Science.
7
The maps
show that the University of Amsterdam is an organisation with a diverse portfolio and
extensive research activity in clinical medicine. Georgia Tech is strong in computer sciences,
materials sciences, and engineering—as well as in applications of engineering, such as
biomedical or environmental technologies. Not surprisingly, LSE’s main activity lies in the
areas of 1) politics, economics and geography, and 2) social studies—with some activity in
the engineering and computer sciences with social applications (e.g. statistics, information
systems, or operations research) and in the health services (e.g. heath care and public
health). To fully appreciate the descriptions, labels for each of the nodes are needed.
Although they are not presented in these figures due to lack of resolution in printed material,
labels can be switched on and off in the computer visualisation interface, as explained in
Appendix 1.
7
For the University of Amsterdam 31,507 publications were retrieved; 26,701 for Georgia Tech; and
6,555 for LSE.

Some of the advantages of overlay maps over local maps are illustrated by Figure 3. First,
they provide a visual framework that enables us to make immediate and intuitively rich
comparisons. Second, they use cognitive units for the representation (disciplines and
specialties) that fit with conventional wisdom, whereas one can expect the analytical
aggregates of local maps to be unstable and difficult to interpret. Third, whereas the
generation of meaningful local maps requires bibliometric expertise, overlay maps can be
produced by users of the Science Citation Index who are not experts in scientometrics.
Finally, they can be used for various purposes depending on the units of analysis displayed
by the size of the nodes: whether number of publications, citing articles, cited references,
growth or other indicators as shown by a series of recent studies (cf. Rafols & Meyer,
forthcoming; Porter & Rafols, 2009; Porter & Youtie, 2009).

U. Amsterdam
Cognitive Sci.
Agri Sci
Biomed Sci
Chemistry
Physics
Engineering
Env Sci & Tech
Matls Sci
Infectious
Diseases
Psychology
Social Studies
Clinical Med
Computer Sci
Business & MGT
GeosciencesEcol Sci
Econ Polit. & Geography
Health & Social Issues
Georgia Tech LSE
Figure 3. Publications profiles of the University of Amsterdam, Georgia Tech and London
School of Economics (LSE) overlaid on the map of science.

6. Conditions of application of the overlay maps
As is the case with all bibliometric indicators, the appropriate use of overlay maps should not
be taken for granted, particularly since they are tools that can be easily used by non-experts
(Gläser & Laudel, 2007). In this section we explore the conditions under which overlay maps
can be valid and useful for science policy analysis and management, building on Rip (1997).
A first issue concerns the use of journals as the basic unit for classification. This is
inaccurate since journals can be expected to combine different epistemic foci, and scientists
can be expected to read sections and specific articles from different journals (Bensman
2007). Furthermore, journal content may not fully match specific categories. In particular,
consider journals such as Nature and Science that cover multiple fields. The ISI includes
these in their category “Multidisciplinary Sciences” (which is factored into our Biomedical
Sciences macro-discipline, even though physics, chemistry, etc., articles appear in it). To
date, we just treat this and the seven other interdisciplinary or multidisciplinary SCs (e.g.,
“Chemistry, Multidisciplinary”) the same as any other SCs.
However, the structural similarity of maps obtained with different classifications suggests that
discrepancies and errors are not biased and therefore tend to average out when aggregated
(Rafols & Leydesdoff, 2009). Hence, the answer to the problem of generalizing from specific
or local data to a global map lies in the power of statistics: given a sufficiently large number
of assignations, there is high probability that the largest number of publications will have
been assigned correctly. Assuming a category with an expected correct assignation of 50%,
the binomial test predicts that about 70 papers are sufficient to guarantee the correct
assignation of at least 40% of the papers to this category with a significance level of 0.05.
Appendix 2 provides further details of the binomial test and estimates of the minimum size of

samples under different constraints.
8
These results suggest that one should be cautious
about asserting how accurately we are “locating” a given body of research based on small
numbers of papers. Instead, for the study of single researchers or laboratories, it may be
best to rely on proxies. For example, if a researcher has 30 publications, the analyst is
advised to consider the set of references within these articles as a proxy for the disciplinary
profile (Rafols & Meyer, forthcoming).
A second set of conditions for the overlay maps to be useful for research policy and
management purposes is transparency and traceability, i.e., being able to specify,
reproduce, and justify the procedures behind the maps in the public domain. Although the
majority of the users of the map may not be interested in the scientometric details, the
possibility to re-trace the methods and challenge assumptions is crucial for the maps to
contribute to policy debates, where transparency is a requirement. For example, Rip (1997)
noted that in the politically charged dispute regarding the ‘decline’ of British science in the
1980s, a key issue of debate concerned the use of static versus dynamic journal categories
(Irvine et al. 1985; Leydesdorff, 1988).
A further requirement for traceability, is relative parsimony, that is, the rule to avoid
unnecessary complexity in procedures and algorithms so that acceptable representations
can be obtained by counter-expertise or even non-experts—even at the expense of some
detailed accuracy—in order to facilitate public discussion, if needed be. In the case of
overlay maps, traceability involves making publicly available the following choices: the
underlying classifications used and/or clustering algorithms to obtain them (in our case, the
ISI SC’s); the similarity measures used among categories (Salton’s cosine similarity); and
8
This result of at least 70 papers for each of the top categories to be identified in an overlay map is
obtained under a very conservative estimate of the accuracy of existing classifications—less stringent
estimates suggest that some 10-20 papers per top category may provide overlay maps with accuracy
within the vicinity of a SC.

the visualisation techniques (Kamada-Kawai with a cosine > 0.15 threshold). These minimal
requirements are needed so that the maps can be reproduced and validated independently.
A third condition of application concerns the appropriateness of the given science overlay
map for the evaluation or foresight questions that are to be answered. Roessner’s (2000)
critique of the indiscriminate use of quantitative indicators in research evaluation applies also
to maps: without a clear formulation of the question of what a programme or an organisation
aims to accomplish, and its context, science maps cannot provide a well-targeted answer.
What type of questions can our overlay maps help to answer? We think that they can be
particularly helpful for comparative purposes in benchmarking collaborative activities and
looking at temporal change, as described in the next section.
7. Use in science policy and research management
The changes that S&T systems are undergoing exacerbate the apparent dissonance
between social and cognitive structures—with new cross-disciplinary or transversal co-
ordinates (Whitley, 2000, p. xl ; Shinn & Ragouet, 2005). As a result, disciplinary labels of
university or R&D units cannot be relied upon to provide an accurate description of their
epistemic activities. This is because researchers often publish outside the field of their
departmental affiliation (Bourke & Butler 1998) and, further, cite outside their field of
publication (Van Leeuwen & Tijssen, 2000)—and increasingly so (Porter & Rafols, 2009).
Science overlay maps offer a method to locate or compare positions, shifts and/or
dissonances in the disciplinary activities at different institutional or thematic levels. This type
of map (with a different basemap) was first introduced by Kevin Boyack and collaborators to
compare the disciplinary differences in the scientific strength of nations,
9
in the publishing
profiles of two large research organisations (Boyack, 2009, pp. 36-37), and the publication
9
http://wbpaley.com/brad/mapOfScience/index.html; accessed December 15, 2009.

outcomes of two funding agencies (Boyack, Börner & Klavans, 2009, p. 49). Some of us
have used previous versions of the current overlay method to
• compare the degree of interdisciplinarity at the laboratory level (Rafols & Meyer,
forthcoming);
• study the diffusion of a research topic across disciplines (Kiss et al., 2009);
• model the evolution over time of cross-disciplinary citations in six established
research fields (SCs -- Porter & Rafols, 2009);
• explore the multidisciplinary knowledge bases of emerging technologies, namely
nanotechnology, as a field (Porter & Youtie, 2009) and specific sub-specialties
(Rafols et al., 2010; Huang et al., forthcoming).
The following examples focus on applications for the purposes of benchmarking,
establishing collaboration and capturing temporal change, as illustrated with universities
(Figure 3), large corporations (Figure 4), funding agencies (Figure 5), and an emergent topic
of research (carbon nanotubes, Figure 6).
10
Benchmarking
A first potential use of organisational comparisons is benchmarking: how is organisation A
performing in comparison to possible competitors or collaborators? For example a
comparison between Pfizer (Figure 4) and Astrazeneca (not shown), reveals at first glance a
very similar profile, centred around biomedical research (pharmacology, biochemistry,
toxicology, oncology) with activity both in clinical medicine and chemistry. However, a more
careful look allows spotting some differences: whereas Pfizer has a strong profile in
nephrology, Astrazeneca is more active in gastroenterology and cardiovascular systems.
10
The data shown were retrieved from Web of Science in October, 2009. Figure 4 is based on 8107
publications by Pfizer, 1772 by Nestlé, 2632 by Unilever, and 1617 by Shell between 2000 and 2009.
Figure 5 is based on 42,440 publications funded by NIH, 40,283 by NSF, 2,104 by BBSRC, and 5,746
by EPSRC, using the new field of “funding agency.” Figure 6 is based on 7,782 publications on
carbon-nanotubes in 2008 (cf. Lucio-Arias and Leydesdorff, 2007).

This description may be too coarse for some purposes (e.g. specific R&D investment), but
sufficient for policy-oriented analysts to discuss the knowledge base of the firms.
11
Several choices can be made regarding the data to be displayed in the maps. First, should
the map display an input (the categories of the papers cited by the organisations), an output
(the categories of a set of publications of the organisation), or an outcome (the categories of
the papers citing the organisation’s research)? Second, should the overlay data be
normalised by the size of the category and/or the size of the organisation? [The figures here
are normalised by the size of the organisation, but not by the size of the category;
normalising by category will bring to the forefront those categories in which one organisation
is relatively very active compared to others, even if it represents a small percentage of its
production.] Third, in addition to the number or proportion of publications per SC (or macro-
discipline), other indicators such as impact factor or growth rate indicators can be mapped
(Noyons et al., 1999; Van Raan & Van Leeuwen, 2002; or Klavans & Boyack, forthcoming).
11
Personal communication with an analyst in a pharmaceutical corporation, November 2009.

Shell
Nestlé
Unilever
Pfizer
Figure 4. Profiles of the publications (2000-2009) of large corporations of different economic
sectors: pharmaceutical (Pfizer), food (Nestlé), consumer products (Unilever), oil (Shell).
Exploring collaborations
A second application of the overlay maps is to explore complementarities and possible
collaborations (Boyack, 2009). For example, Nestlé’s core activities lie in food-related
science and technology. Interestingly, the map reveals that one of its areas of highest
research publication activity, the field of nutrition and dietetics (the dark green spot in the
light green cluster in Figure 4 for Nestlé), falls much closer to the biomedical sciences than
other food-related research. This suggests that the field of nutrition may act as bridge and
common ground for research collaboration between the food and pharmaceutical industry—
sectors that are approaching one another, as shown by Nestlé’s strategic R&D investment in
‘functional’ (i.e. health-enhancing) foods (The Economist, 2009).

In Figure 5, we compare funding agencies in terms of potential overlap. The funding
agencies in the US and the UK have, in principle, quite differentiated remits. In the US (top
of Figure 5), the NIH (National Institute of Health) focuses on biomedical research while the
NSF (National Science Foundation) covers all basic research. In the UK (bottom of Figure
5), the BBSRC (Biotechnology and Biological Sciences Research Council) and the EPSRC
(Engineering and Physical Sciences Research Council) are expected to cover the areas
described in their respective names. However, Figure 5 reveals substantial areas of overlap.
These are areas where duplication of efforts could be occurring—suggesting a case for
coordination among agencies. It may also help indentify interdisciplinary topics warranting
express collaboration between committees from two agencies, such as the interaction of the
BBSRC and EPSRC on Engineering and Biological Systems.
The exploration of collaboration practices is a topic where overlay maps provide added value
because they implicitly convey information regarding the cognitive distance among the
potential collaborators. A variety of studies (Llerena & Meyer-Krahmer, 2004; Cummings &
Kiesler, 2005; Noteboom et al., 2007; Rafols, 2007) have suggested that successful
collaborations tend to occur in a middle range of cognitive distance, whereupon the
collaborators can succeed at exchanging or sharing complementary knowledge or
capabilities, while still being able to understand and coordinate with one another. At short
cognitive distances, the benefits of collaboration may be too low to be worth the effort (or
competition may be too strong), while at large distances, understanding between partners
may become difficult. It remains an empirical question whether one may think of an ‘optimal
cognitive distance’ which would allow formulating a research project with ‘optimal diversity’
(Van den Bergh, 2008).
In any case, overlay maps offer a first (yet crude) method to explore complementarities
between prospective partners. US managers of grant programmes for highly innovative
research pointed out to us that the science overlay maps might be useful for finding partners,

as well as for evaluating prospective grantees. The U.S. National Academies Keck Futures
Initiative (NAKFI) has found it helpful to overlay research publications pertaining to a
prospective workshop topic (synthetic biology) to help identify research communities to
include.
NIH NSF
BBSRC EPSRC
Figure 5. Publication profile of funding agencies in the US (top) and the UK (bottom).
Capturing temporal change
A third use of overlap maps is to compare developments over time. This allows exploring the
diffusion of research topics across disciplines (Kiss et al., 2009). In cases where the
research topic is an instrument, a technique or a research material, the spread may cover
large areas of the science map (as noted by Price, 1984, p. 16). Figure 6 shows the location
of publications on carbon-nanotubes (left) and its areas of growth (right). The growth rate
was computed by calculating the annual growth between 2004 and 2008 and taking the
average over the period. Since their discovery in 1991, carbon-nanotubes research has

shown exponential growth, first in the areas of materials sciences and physical chemistry
(Lucio-Arias & Leydesdorff, 2007). However, nowadays the highest growth can be observed
in computer sciences due to electronic properties of carbon-nanotubes (pink), in medical
applications (red: e.g., imaging and biomedical engineering), and both in biomedical
research (green: e.g. pharmacology and oncology) and in environmental research (orange).
Within the dominant areas of chemistry and materials sciences (blue and black), growth is
highest in applied fields, such as materials for textiles and biomaterials. The overlay
methodology thus offers a perspective of the shift of carbon-nanotubes research towards
applications and issues of health and environmental safety. Alternatively to a static display of
growth rate, the overlay maps can make a “movie” of the evolution of a field (e.g., via a
succession of Powerpoint time-slice slides; this works because of the stable basemap).
Comparison over time can also be interesting in order to track developments in
organisations. For example, Georgia Tech, traditionally an engineering-centred university
without a medical school, recently created the School of BioMedical Engineering. Going
back to Figure 3, we can see a medium-size red spot in Georgia Tech publications
corresponding to biomedical engineering. A dynamic analysis would depict how this has
grown in the last decade.
Since the rationales of research policy, evaluation and management are more complex than
bibliometric indicators or maps can be, science overlay maps will usually provide
complementary inputs to support (and sometimes to justify) decisions. Other possible uses
include finding reviewers for the assessment of interdisciplinary research in emergent fields,
or finding valid benchmarks when comparing organisations (Laudel & Gläser, 2009).

Publications
(2008)
Average Growth
(2004-2008)
Figure 6. Publications (2008) and average annual growth of publications (2004-2008) on
carbon nanotubes.
8. Advantages and limitations of overlays
We noted above as some major advantages and downsides of overlay maps: on the plus
side, their readability, intuitive and heuristic nature; on the minus side, the inaccuracy in the
attribution to categories and the possible error by visual inspection of cognitive distance given
the reduction of dimensions. In this section, we explore further potential benefits of maps in
terms of cognitive contextualisation and capturing diversity, and its main limitation, namely its
lack of local relational structures.
Contextualising categories
Science overlay maps provide a concise way to contextualise previously existing information
of an organisation or topic, in a cognitive space. The same information overlaid on the maps
may well have been provided in many previous studies in tabular or bar chart format. For
example, policy reports (e.g., Van Raan & Van Leeuwen, 2002) may extensively show the
outcomes of a research programme via tables and bar charts: fields of publication, user
fields, relative impacts, changes of these indicators over time, etc. What would the overlay
maps offer more than this? In our opinion, these maps provide the contextualisation of the
data. This extension not only facilitates the comprehension of sets of data, but also their

correct interpretation. Unlike bar charts and tables based on categories, the overlay maps
remain valid (statistically acceptable) despite possible errors in the classifications. The
reason is that, whereas different classifications may produce notably different bar charts, in
corresponding maps ‘misclassified’ articles fall in nearby nodes and the user may still be
provided with an adequate pattern. The context can thus reduce perceptual error.
For example, let us consider the new ISI SC of Nanoscience and Nanotechnology. A study
of a university department in materials science during the 2000s might suggest a strong shift
towards nanotechnology based on considering bar charts that show its strong growth in this
new SC. However, on a global map of science, this new SC, Nanoscience and
Nanotechnology, locates extremely closely to other core disciplines in Materials Sciences.
Therefore, one would appreciate this change as a relatively small shift in focus, rather than a
major cognitive shift. If a department under study had fully ventured into more
interdisciplinary nanotechnology, its publications would also increasingly be visible in more
disparate disciplines, for example, in the biomedical or environmental areas (Porter & Youtie,
2009).
Capturing diversity
Science overlay maps provide the user with a perspective of the disciplinary diversity of any
given output, yet without the need to rely on combined or composite indices. Research
organisations often seek a diverse cognitive portfolio, but find it difficult to assess whether
the intended diversity is achieved. However, diversity encapsulates three entangled aspects
(variety, balance, and disparity) which cannot be univocally subsumed under a single index
(Stirling, 2007), but are differently reflected in these maps:
• First, the maps capture the variety of disciplines by portraying the number of
disciplines (nodes) in which a research organisation is engaged;
• Second, they capture the disciplinary balance by plotting the different sizes of the SC
nodes;

• Third—different from, say, bar charts—maps can convey the disparity (i.e. the
cognitive distances) among disciplines by placing these units closer or more distant
on the map (Rafols & Meyer, forthcoming).
This spatial elaboration of diversity measures is particularly important when comparing
scientific fields in terms of multi- or interdisciplinarity. For example, Porter & Rafols (2009)
show that in fields such as biotechnology, many disciplines are cited (high variety, a mean of
12.7 subject categories cited per article in 2005), but they are mainly cited in the highly
dense area around biomedical sciences (low disparity). In contrast, atomic physics
publications cite fewer disciplines (a mean of 8.7 per article), but from a more diverse
cognitive area, ranging from physics to materials science and chemistry (higher disparity).
This discussion highlights that overlay maps are useful to explore interdisciplinary
developments. In addition to capturing disciplinary diversity, they can also help to clarify the
relative location of disciplines and thereby enable us to gain insights of another of the
aspects of interdisciplinary research, namely their position in between or central (or
marginal) to other research areas (Leydesdorff, 2007). Unlike indicators that seek to digest
multiple facets to a single value or ranking of the extent of “interdisciplinarity,” maps invite
the analyst to more reflexive explorations and provide a set of perspectives that can help to
open the debate. This plurality is highly commendable given the conspicuous lack of
consensus on the assessment of interdisciplinarity (Rinia et al. 2001, Morillo et al., 2003;
Bordons et al., 2004; Leydesdorff, 2007; Porter et al. 2007; Rafols & Meyer, forthcoming;
see review by Wagner et al., submitted).
Missing the relational structure
The two characteristics that make overlay maps so useful for comparisons, their fixed
positional and cognitive categories, are also inevitably, their major limitations and a possible
source of misreading. Since the position in the map is only given by the attribution in the

disciplinary classification, the resulting map does not teach us anything about the direct
linkages between the nodes. For example, Figure 3 shows that the University of Amsterdam
covers many disciplines—but we do not know at all whether its local dynamics is organised
within the disciplines portrayed or according to a variety of themes transversal to a collection
of SCs. In order to investigate this, one would need to create local maps, as described in
Section 3. For most local purposes these maps will be based on smaller units of analysis,
such as words, publications or journals, rather than SCs.
In our opinion, a particularly helpful option is to combine overlay maps (based on a top-down
approach, with fixed and given categories) with local maps (based on a bottom-up approach,
with emergent structures), in order to capture the dynamics of an evolving field (Rafols &
Meyer, forthcoming; Rafols et al., 2010; Rosvall & Bergstrom, 2009). A recursive combination
of overlay and local maps allows us to investigate the evolution of a field both in terms of its
internal cognitive coherence and the diversity of its knowledge sources with reference to
disciplinary classifications (external).
9. Conclusions
Science overlay maps offer a straightforward and intuitive way of visualising the position of
organisations or topics in a fixed map based on conventional disciplinary categories. By
thus standardizing the mapping, one can produce comparisons which are easy to grasp for
science managers or policy-makers. For example, one can assess a research portfolio of a
university or animate a diffusion pattern of an emergent field.
In this study, we have introduced the bases for the use of overlay maps to prospective non-
expert users and described how to create them. We demonstrated that the emergent
consensus on the structure of science enables us to generate and warrant a stable global
template to use as a basemap. We introduced the conditions to be met for a proper use of

the maps, including a sample size of statistical reliability, and the requirements of
transparency and traceability. We provided examples of benchmarking, search of
collaborations and examination of temporal change in applications to universities,
corporations, funding agencies and emergent topics.
In our opinion, overlay maps provide significant advantages in the readability and
contextualisation of disciplinary data and in the interpretation of cognitive diversity. As it is
the case with maps in general, overlays are more helpful than indicators to accommodate
reflexive scrutiny and plural perspectives. Given the potential benefits of using overlay maps
for research policy, we provide the reader with an interactive webpage to explore overlays
(http://idr.gatech.edu/maps) and a freeware-based toolkit (available at
http://www.leydesdorff.net/overlaytoolkit ).
Acknowledgements
We are very grateful to Ziyo Wang for her enthusiastic design of the interactive webpages.
We are indebted to K. Boyack, R. Klavans, F. de Moya-Anegón, B. Vargas-Quesada and M.
Zitt for insights and discussions. ALP and IR acknowledge support from the US National
Science Foundation (Award #0830207, ‘Measuring and Tracking Research Knowledge
Integration’, http://idr.gatech.edu/ ). The findings and observations contained in this paper
are those of the authors and do not necessarily reflect the views of the National Science
Foundation. IR is partially supported by the EU FP7 project FRIDA, grant 225546.
References
Abbot, A. (2001). Chaos of disciplines. Chicago, The University of Chicago Press.
Balaban, A. T. & Klein, D. J. (2006). Is chemistry "The Central Science"? How are different
sciences related? Cocitations, reductionism, emergence, and posets. Scientometrics,
69, 615-637.
Bapteste, E., O'Malley, M. A., Beiko, R. G., Ereshefsky, M., Gogarten, J. P., Franklin-Hall, L.,
Lapointe, F.-J., Dupré, J., Dagan, T., Boucher, Y. & Martin, W. (2009). Prokaryotic
evolution and the tree of life are two different things. Biology Direct, 4(34), 1-20.
Bensman, S. J. (2007). Garfield and the impact factor. Annual Review of Information
Science and Technology, 41(1), 93-155.
Bensman, S. J., & Leydesdorff, L. (2009). Definition and Identification of Journals as
Bibliographic and Subject Entities: Librarianship vs. ISI Journal Citation Reports
(JCR) Methods and their Effect on Citation Measures. Journal of the American
Society for Information Science and Technology, 60(6), 1097-1117.
Bollen, J., Van de Sompel, H., Hagberg, A., Bettencourt, L., Chute, R., Rodriguez, M. A. and
Balakireva, L. (2009). Clickstream data yields high-resolution maps of science. PLoS
ONE 4(3), e4803.

Bordons, M., Morillo, F. and Gómez, I. (2004). Analysis of cross-disciplinary research
through bibliometric tools. Handbook of quantitative science and technology
research. H. F. Moed, W. Glänzel and U. Schmoch. Dordrecht, Kluwer: 437-456.
Börner, K., Chen, C. and Boyack, K. W. (2003). Visualizing knowledge domains. Annual
Review of Information Science & Technology 37, 179-255.
Bourke, P. & Butler, L. (1998). Institutions and the map of science: matching university
departments and fields of research. Research Policy, 26, 711–718.
Bowker, G. C. and Star, S. L. (2000). Sorting things out: Classification and its
consequences. Cambridge, MA, MIT Press.
Boyack, K. W. (2009). Using detailed maps of science to identify potential collaborations.
Scientometrics, 79(1), 27-44.
Boyack, K. W., Klavans, R. & Börner, K. (2005). Mapping the backbone of science.
Scientometrics, 64(3), 351-374.
Boyack, K. W., Börner, K. & Klavans, R. (2009). Mapping the structure and evolution of
chemistry research. Scientometrics, 79(1), 45-60.
Callon, M., Law, J. and Rip, A., Eds. (1986). Mapping the dynamics of science and
technology : Sociology of science in the real world. London, MacMillan.
Cambrosio, A., Keating, P., Mercier, S., Lewison, G. & Mogoutov, A. (2007). Mapping the
emergence and development of translational cancer research. European Journal of
Cancer, 42, 3140-3148.
Cummings, J. N. & Kiesler, S. (2005). Collaborative research across disciplinary and
organizational boundaries. Social Studies of Science, 35(5), 733-722.
Dupré, J. (1993). The Disorder of Things. Metaphysical Foundations of the Disunity of
Science. Cambridge, MA: Harvard University Press.
Etzkowitz, H. & Leydesdorff, L. (2000). The dynamics of innovation: from National Systems
and "Mode 2" to a Triple Helix of university-industry-government relations. Research
Policy, 29, 109-123.
Galison, P. & Stump, D. J., Eds. (1996). The disunity of science: boundaries, context and
power. Stanford, CA, Stanford University Press.
Garfield, E. (1987). Launching the ISI Atlas of Science: for the new year, a new generation of
reviews. Current Contents, 1, 1-6.
Gibbons, M., Limoges, C., Nowotny, H., Schwartzman, S., Scott, P. & Trow, M. (1994). The
New Production of Knowledge: The Dynamics of Science and Research in
Contemporary Societies. London, Sage.
Gläser, J. and Laudel, G. (2007). The social construction of bibliometric evaluations. The
changing governance of the sciences. R. Whitley and J. Gläser. Amsterdam,
Springer: 101-123.
Hessels, L. K. and Van Lente, H. (2008). Re-thinking new knowledge production: A literature
review and a research agenda. Research Policy 37, 740-760.
Hicks, D. (1987). Limitations of co-citation analysis as a tool for science policy. Social
Studies of Science 17, 295-316.
Huang, L., Guo, Y., & Porter, A.L. (forthcoming). Identifying Emerging Roles of Nanoparticles
in Biosensors, Journal of Business Chemistry.
Irvine, J., Martin, B., Peacock, T., & Turner, R. (1985). Charting the decline of British
science. Nature, 316, 587-590.
Janssens, F., Zhang, L., Moorc, B. D. and Glänzel, W. (2009). Hybrid clustering for
validation and improvement of subject-classification schemes. Information
Processing and Management 45(6), 683-702.
Katz, J. S. (1999). The self-similar science system. Research Policy 28(5), 501-517
Kamada, T., & Kawai, S. (1989). An algorithm for drawing general undirected graphs.
Information Processing Letters, 31(1), 7-15.
Kiss, I. Z., Broom, M., Craze, P. & Rafols, I. (2010). Can epidemic models describe the
diffusion of research topics across disciplines? Journal of Informetrics, DOI:
10.1016/j.joi.2009.08.002.

Klavans, R. & Boyack, K. W. (2008). Thought leadership: A new indicator for national and
institutional comparison. Scientometrics, 75(2), 239-250.
Klavans, R. & Boyack, K. W. (2009). Toward a Consensus Map of Science. Journal of the
American Society for Information Science and Technology, 60(3), 455-476.
Klavans, R. & Boyack, K. W. (forthcoming). Toward an objective, reliable and accurate
method for measuring research leadership. Scientometrics.
Klein, J. T. (2000). A conceptual vocabulary of interdisciplinary science. In Practising
interdisciplinarity. P. Weingart & N. Stehr, Eds. Toronto, University of Toronto Press:
3-24.
Knorr-Cetina, K. (1999). Epistemic cultures. How the sciences make knowledge. Cambridge,
MA, Harvard University Press.
Laudel, G. & Gläser, J. (2009). Interdisciplinarity as criterion and condition of evaluations.
Paper presented at the Workshop on Interdisciplinarity in the 2009 ISSI Conference,
Rio de Janeiro, July 2009.
Lenoir, T. (1997). Instituting science. The cultural production of scientific disciplines.
Stanford, Stanford University Press.
Leydesdorff, L. (1986). The Development of Frames of References. Scientometrics 9, 103-
125.
Leydesdorff, L. (1987). Various methods for the Mapping of Science. Scientometrics 11,
291-320.
Leydesdorff, L. (1988). Problems with the 'measurement' of national scientific performance.
Science and Public Policy, 15, 149-152.
Leydesdorff, L. (2006). Can Scientific Journals be Classified in Terms of Aggregated
Journal-Journal Citation Relations using the Journal Citation Reports? Journal of the
American Society for Information Science & Technology, 57(5), 601-613.
Leydesdorff, L. (2007). Betweenness centrality as an indicator of the interdisciplinarity of
scientific journals. Journal of the American Society for Information Science &
Technology, 58(9), 1303-1319.
Leydesdorff, L., Cozzens, S. E., & Van den Besselaar, P. (1994). Tracking Areas of Strategic
Importance using Scientometric Journal Mappings. Research Policy, 23, 217-229.
Leydesdorff, L. & Rafols, I. (2009). A Global Map of Science Based on the ISI Subject
Categories. Journal of the American Society for Information Science & Technology,
60(2), 348-362.
Llerena, P. & Meyer-Krahmer, F. (2004). Interdisciplinary research and the organization of
the university: general challenges and a case study. Science and Innovation. In
Rethinking the Rationales for Funding and Governance. A. Geuna, A. J. Salter and
W. E. Steinmueller. Eds. Cheltenham, Edward Elgar: 69-88.
Lucio-Arias, D. & Leydesdorff, L. (2007). Knowledge emergence in scientific communication:
From “fullerenes” to “nanotubes”. Scientometrics 70(3), 603-632.
Martin, B.R. (1997). The use of multiple indicators in the assessment of basic research.
Scientometrics, 36(3), 343-362.
Maturana, H. R., & Varela, F. J. (1984). The Tree of Knowledge. Boston: New Science
Library.
Morillo, F., Bordons, M. & Gómez, I. (2003). Interdisciplinarity in science: A tentative
typology of disciplines and research areas. Journal of the American Society for
Information Science and Technology, 54(13), 1237 - 1249.
Moya-Anegón, F., Vargas-Quesada, B., Victor Herrero-Solana, Chinchilla-Rodríguez, Z.,
Elena Corera-Álvarez & Munoz-Fernández, F. J. (2004). A new technique for building
maps of large scientific domains based on the cocitation of classes & categories.
Scientometrics, 61(1), 129-145.
Moya-Anegón, F., Vargas-Quesada, B., Chinchilla-Rodríguez, Z., Corera-Álvarez, E.,
Munoz-Fernández, F. J. & Herrero-Solana, V. (2007). Visualizing the marrow of
science. Journal of the American Society for Information Science & Technology, 58,
2167–2179.

NAS (2009) Data on Federal Research and Development Investments: A Pathway to
Modernization. National Academies Press, Washington D.C. Available at
http://www.nap.edu/catalog/12772.html, accessed November 20
th
, 2009.
Neurath, O. (1932/1933). Protokollsätze. Erkenntnis, 3, 204-214.
Nooteboom, B., Haverbeke, W. V., Duysters, G., Gilsing, V. & van den Oord, A. (2007).
Optimal cognitive distance and absorptive capacity. Research Policy 36, 1016–1034.
Noyons, E. (2001). Bibliometric mapping of science in a science policy context.
Scientometrics, 50(1), 83-98.
Noyons, C. M. (2004). Science maps within a science policy context. Handbook of
quantitative science & technology research. H. F. Moed, W. Glänzel & U. Schmoch.
Dordrecht, Kluwer: 237-255.
Noyons, E. C. M. and Raan, A. F. J. v. (1998). Monitoring scientific developments from a
dynamic perspective: Self-organized structuring to map neural network research.
Journal of the American Society for Information Science 49, 68-81.
Noyons, E. C. M., H. Moed & M. Luwel (1999). Combining mapping and citation analysis for
evaluative bibliometric purposes. Journal of the American Society for Information
Science 50(2), 115-131.
Porter, A. L., Cohen, A. S., Roessner, J. D. & Perreault, M. (2007). Measuring researcher
interdisciplinarity. Scientometrics, 72(1), 117-147.
Porter, A. L. & Rafols, I. (2009). Is Science Becoming more Interdisciplinary? Measuring and
Mapping Six Research Fields over Time. Scientometrics, 81(3), 719-745.
Porter, A.L., & Youtie, J. (2009). Where Does Nanotechnology Belong in the Map of
Science?, Nature-Nanotechnology, 4, 534-536.
Price, D. J. de Solla (1965). Networks of scientific papers. Science, 149, 510- 515.
Price, D. J. de Solla (1984). The science/technology relationship, the craft of experimental
science, and policy for the improvement of hight technology innovation. Research
Policy 13, 3-20.
Pudovkin, A. I. and Garfield, E. (2002). Algorithmic procedure for finding semantically related
journals. Journal of the American Society for Information Science and Technology
53(1113-1119), 11.
Rafols, I. (2007). Strategies for knowledge acquisition in bionanotechnology: Why are
interdisciplinary practices less widespread than expected?. Innovation: the European
Journal of Social Science Research, 20(4), 395-412.
Rafols, I. & Leydesdorff, L. (2009). Content-based and Algorithmic Classifications of
Journals: Perspectives on the Dynamics of Scientific Communication and Indexer
Effects. Journal of the American Society for Information Science & Technology, 60(9)
1823-1835.
Rafols, I. & Meyer, M. (forthcoming). Diversity and Network Coherence as indicators of
interdisciplinarity: case studies in bionanoscience. Scientometrics, DOI
10.1007/s11192-009-0041-y.
Rafols, I., Park, J.-H. & Meyer, M. (2010). Hybrid nanomaterial research: is it really
interdisciplinary? In The Supramolecular Chemistry of Organic- Inorganic Hybrid
Materials. K. Rurack & R. Martínez- Máñez. Eds. Hoboken, NJ, US, John Wiley &
Sons.
Rinia, E. J., Van Leeuwen, T. N., Bruins, E. P. W., Van Buren, H. G. & Van Raan, A. F. J.
(2002). Measuring knowledge transfer between fields of science. Scientometrics, 54,
347–362.
Rip, A. (1997). Qualitative conditions of scientometrics: The new challenges. Scientometrics
38(1), 7-26.
Roessner, D. (2000). Quantitative and qualitative methods and measures in the evaluation of
research Research Evaluation 9(2), 125-132.
Rosvall, M. & Bergstrom, C. T. (2009). Mapping change in large networks.
arXiv:0812.1242v1.

Small, H. G. (1977). A co-citation model of a scientific specialty: A longitudinal study of
collagen research. Social Studies of Science 7, 139-166.
Simon, H. A. (1973). The Organization of Complex Systems. In H. H. Pattee (Ed.), Hierarchy
Theory: The Challenge of Complex Systems (pp. 1-27). New York: George Braziller
Inc.
Shinn, T. & Ragouet, P. (2005). Controverses sur la science. Pour une sociologie
transversaliste de l'activité scientifique. Paris, Raisons d'agir.
Small, H. (1973). Co-citation in the Scientific Literature: A New measure of the Relationship
between Two Documents. Journal of the American Society for Information Science,
24(4), 265-269.
Small, H. (1999). Visualizing science by citation mapping. Journal of the American Society
for Information Science 50(9), 799-813.
Small, H., & Griffith, B. (1974). The Structure of Scientific Literature I. Science Studies 4, 17-
40.
Small, H., & Sweeney, E. (1985). Clustering the Science Citation Index Using Co-Citations I.
A Comparison of Methods,. Scientometrics 7, 391-409.
Small, H., Sweeney, E., & Greenlee, E. (1985). Clustering the Science Citation Index Using
Co-Citations II. Mapping Science,. Scientometrics 8, 321-340.
Weingart, P. and Stehr, N., Eds. (2000). Practising interdisciplinarity. Toronto, University of
Toronto Press.
Stirling, A. (2007). A general framework for analysing diversity in science, technology and
society. Journal of The Royal Society Interface, 4(15), 707-719.
Stirling, A. (2008). "Opening up" And "Closing down": Power, participation, and pluralism in
the social appraisal of technology. Science, Technology & Human Values 33(2), 262-
294.
Studer, K. E., & Chubin, D. E. (1980). The Cancer Mission. Social Contexts of Biomedical
Research. Beverly Hills.: Sage.
The Economist (2009) Nestlé. The unrepentant chocolotier. October 29th 2009.
Tijssen, R., de Leeuw, J., & van Raan, A. F. J. (1987). Quasi-Correspondence Analysis on
Square Scientometric Transaction Matrices. Scientometrics 11, 347-361.
Van den Bergh, J. C. J. M. (2008). Optimal diversity: Increasing returns versus recombinant
innovation. Journal of Economic Behavior & Organization 68, 565-580.
van den Besselaar, P. and Leydesdorff, L. (1996). Mapping change in scientific specialties:
A scientometric reconstruction of the development of artificial intellingence. Journal of
the American Society for Information Science 46(6), 415-436.
Van den Daele, W., Krohn, W., & Weingart, P. (Eds.). (1979). Geplante Forschung:
Vergleichende Studien über den Einfluss politischer Programme auf die
Wissenschaftsentwicklung. Frankfurt a.M.: Suhrkamp.
Van Eck, N.J. & Waltman, L. (2009) VOSviewer: A computer program for bibliometric
mapping. In B. Larsen & J. Leta, editors, Proceedings of the 12th International
Conference on Scientometrics & Informetrics, pp. 886-897.
Van Leeuwen, T. & Tijssen, R. (2000). Interdisciplinary dynamics of modern science:
Analysis of cross-disciplinary citation flows. Research Evaluation, 9(3), 183-187.
Van Raan, A. F. J. & van Leeuwen, T. N. (2002). Assessment of the scientific basis of
interdisciplinary, applied research. Application of bibliometric methods in Nutrition
and Food Research. Research Policy 31, 611-632.
Wagner, C. S., Roessner, J. D., Bobb, K., Klein, J. T., Boyack, K. W., Keyton, J., Rafols, I. &
Börner, K. (Submitted). Approaches to Understanding and Measuring
Interdisciplinary Scientific Research (IDR): A Review of the Literature. Journal of
Informetrics.
Weingart, P. & Stehr, N., Eds. (2000). Practising Interdisciplinarity. Toronto, University of
Toronto Press.
Weingart, P. (2000). Interdisciplinarity: the paradoxical discourse. In Practising
interdisciplinarity. P. Weingart & N. Stehr. Eds. Toronto, University of Toronto Press:
25-41.

White, H. D. and McCain, K. W. (1998). Visualizing a discipline: An author co-citation
analysis of information science, 1972-1995. Journal of the American Society for
Information Science 49(9), 327-355.
Whitley, R. (2000). The intellectual and social organisation of the sciences. Oxford, Oxford
Universidy Press, 2nd Edition.
Woolgar, S. (1991). Beyond the citation debate: Towards a sociology of measurement
technologies and their use in science policy. Science and Public Policy 18(5), 319-
326.

Appendix 1: A user-friendly method for the generation of overlay maps
We follow the method introduced in Rafols & Meyer (forthcoming) to create the overlay map
on the basis of a global map of science (Leydesdorff & Rafols, 2009). The steps described
below rely on access to the Web of Science and the files available in our mapping kit
(http://www.leydesdorff.net/overlaytoolkit). The objective is to obtain the set of SCs for a
given set of articles; provide this to network software (we describe for Pajek); and output as
overlay information to add to a suitable basemap.
First, the analyst has to conduct a search in the Thomson Reuters Web of Science
(www.isiknowledge.com). Non-expert users should note that this initial step is crucial and
should be done carefully: authors may come with different initials, addresses are often
inaccurate, and only some types of document ,may be of interest (e.g., only so-called citable
items: articles, proceedings papers, reviews, and letters). Once the analyst has chosen a set
of documents from searches at Web of Science, one can click the tab, Analyze results. In
this new webpage, the selected document set can then be analysed along various criteria
(top left hand tab). The Subject Area choice produces a list with the number of documents in
each Subject Category. This list can be downloaded as Analyze.txt. In the next step the
analyst can go to our webpage for maps (http://idr.gatech.edu/maps ) and upload this file .
If one analyst desires more control on the process, she can use the programme Pajek and
the associated overlaytoolkit. After opening Pajek, press F1 and upload the basemap file
SC2007-015cut-2D-KK.paj. This files provide the basemap,
12
as shown by selecting
Draw>Draw-Partition-Vector (or pressing Ctrl-P). Then the previously downloaded
Analyze.txt file has to be transformed by the mini-programme SC2007.exe (in our tool kit) as
into the Pajek vector format “SC07.vec” This file can be uploded into Pajek by choosing
12
The matrix underlying the basemap and the grouping of SCs is available at:
http://www.leydesdorff.net/overlaytoolkit/sc2007.xls

File>Vector>Read from the main Pajek menu. Selecting from the menu Draw>Draw-
Partition-Vector (alternatively, pressing Ctrl-Q), the overlay map will be generated. At this
stage, the size of nodes will often need adjustment, which can be done by selecting
Options>Sizeof Vertices in the new draw window. In order to have the standard colour
settings, the file SC2007-18Factors-ColourSettings.ini can be loaded by going to Options>Ini
File>Load in the main Pajek window. Crtl-L and Ctrl-D allow visualise and delete,
respectively, the labels of each SC. Clickling on nodes allows to move SC to other positions.
The image can be exported selecting Export>2D>Bitmap in the menu of the Draw window. A
further optional step would be to label the map in terms of factors, by importing this image
into powerpoint in order to label groups of clusters, as shown in the file SC2007 Global
maps.ppt.
An alternative procedure for more experienced users is to download the records of a
document set found in the Web of Science. This is done by adding the Marked list (bottom
bar) the desired documents; second, going to Marked list (top bar) and then downloading the
documents in a Tagged Field format after selecting Subject category as one of the fields to
download. The downloaded file should be renamed as data.txt and used as input into the
program ISI.exe (available at http://www.leydesdorff.net/software/isi). One of the outputs of
the programmes ISI.exe is the file SC07.vec that can be used in Pajek as explained above.
The advantage of this procedure is that ISI.exe also produces other files with information on
fields such as author or journal that may be of interest. Feel free to contact the authors in
case of difficulty.

Appendix 2: Estimation of number of papers needed for reliability in overlay maps
In a previous study, Rafols & Leydesdorff (2009) found that there is between 40-60% of
disagreements between attributions of journals to disciplinary categories. Taking a
conservative approach, let us assume that for a sample of N papers, there is a probability
p=0.5 that they will be misclassified by a given classification (whatever the one that is used).
How large should a sample of papers be so that, in spite of the error, the largest categories
in the distribution correctly represent the core discipline of the population?
Let us then assume that we have N papers of one given category A. Given the p=0.5
probability of correct assignation, we only expect 50% of the papers in category A. The
analyst has then to arbitrarily choose a lower threshold m (we suggest 40%), as the
minimum percentage acceptable, with a given degree of significance (we suggest =0.05,
corresponding to a z-score of 1.65). Since a given paper can either be correctly or incorrectly
assigned to a category, we can use the binomial distribution to make a binomial test. For N ≥
50 and Np(1 – p) ≥ 9, the binomial distribution can be approximated to the normal
distribution, with the following z-score:
)1(
)(
pNp
mpN
z
−
−
=
For a given degree of significance , the associated z-score allows us to calculate then the
minimum size of the population N to guarantee that the correct category A will have at least
a proportion m in the sample.
)1(
2
pp
mp
z
N
critical
−






−
≥

Assuming a degree of mis-classification of 50%, enforcing a lower acceptance threshold of
40% and a significance level of =0.05, one obtains that the sample should be larger than
approximately 70 papers (140 papers would increase the significance level to =0.01).
Table 1 shows the number of papers needed under different assumptions.
Table 1. Approximate number of papers recommended for the reliable identification of a
category in an overlay map
p
(Prob.
disagreement)
m
(Lower tolerance)

(Significance level)
Minimuml number of
papers needed
0.5 0.4 0.10 41
0.5 0.4 0.05 68
0.5 0.4 0.01 135
0.6 0.5 0.05 65
0.4 0.3 0.05 65
These results teach us the number of papers N needed in the populated categories to have
some certainty. This means that the actual number of papers per overlay map depends on
how narrow or wide is the distribution of disciplinary categories. The more skewed the
distribution, the fewer papers are needed. Taking the example of Figure 3, one can estimate
that in diverse universities such as Amsterdam or Sussex 3,000 publications may be needed
to capture precisely the five top disciplines, whereas for focused organisations such as the
European Molecular Biology Laboratory (EMBL), 1,500 publications could be enough.
In our opinion, these are rather conservative estimates, having set at p=0.5 of mis-
assignation. If one allows for ‘near-misses’ (i.e. assignation to the two nearest categories to
be counted as correct) then p can be estimated in the range of 0.70 to 0.85 (Rafols &
Leydesdorff, 2009). In this case only some dozens of papers are needed to achieve m~0.5.
(but in this case, the normal distribution approximation cannot be used for the estimate).