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Five steps to conducting a systematic review
Khalid S Khan MB MSc Regina Kunz MD MSc
1
Jos Kleijnen MD PhD
2
Gerd Antes PhD
3
J R Soc Med 2003;96:118–121
Systematic reviews and meta-analyses are a key element of
evidence-based healthcare, yet they remain in some ways
mysterious. Why did the authors select certain studies and
reject others? What did they do to pool results? How did a
bunch of insignificant findings suddenly become significant?
This paper, along with a book
1
that goes into more detail,
demystifies these and other related intrigues.
A review earns the adjective systematic if it is based on a
clearly formulated question, identifies relevant studies,
appraises their quality and summarizes the evidence by use
of explicit methodology. It is the explicit and systematic
approach that distinguishes systematic reviews from
traditional reviews and commentaries. Whenever we use
the term review in this paper it will mean a systematic review.
Reviews should never be done in any other way.
In this paper we provide a step-by-step explanation—
there are just five steps—of the methods behind reviewing,
and the quality elements inherent in each step (Box 1). For
purposes of illustration we use a published review
concerning the safety of public water fluoridation, but we
must emphasize that our subject is review methodology, not
fluoridation.
EXAMPLE: SAFETY OF PUBLIC WATER
FLUORIDATION
You are a public health professional in a locality that has
public water fluoridation. For many years, your colleagues
and you have believed that it improves dental health.
Recently there has been pressure from various interest
groups to consider the safety of this public health intervention
because they fear that it is causing cancer. Public health
decisions have been based on professional judgment and
practical feasibility without explicit consideration of the
scientific evidence. (This was yesterday; today the evidence is
available in a York review
2,3
, identifiable on MEDLINE
through the freely accessible PubMed clinical queries
interface [http://www.ncbi.nlm.nib.gov/entrez/query/
static/clinical.html], under ‘systematic reviews’.)
STEP 1: FRAMING THE QUESTION
The research question may initially be stated as a query in
free form but reviewers prefer to pose it in a structured and
explicit way. The relations between various components of
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JOURNAL OF THE ROYAL SOCIETY OF MEDICINE Volume 96 March 2003
Education Resource Centre, Birmingham Women’s Hospital, Birmingham B15
2TG, UK;
1
German Cochrane Centre, Freiburg and Department of Nephrology,
Charite
´
, Berlin, Germany;
2
Centre for Reviews and Dissemination, York, UK;
3
German Cochrane Centre, Freiburg, Germany
Correspondence to: Khalid S Khan
E-mail: khalid.khan@bham-womens.thenhs.com
Step 1: Framing questions for a review
The problems to be addressed by the review should be
specified in the form of clear, unambiguous and structured
questions before beginning the review work. Once the review
questions have been set, modifications to the protocol should
be allowed only if alternative ways of defining the populations,
interventions, outcomes or study designs become apparent
Step 2: Identifying relevant work
The search for studies should be extensive. Multiple resources
(both computerized and printed) should be searched without
language restrictions. The study selection criteria should flow
directly from the review questions and be specified apriori.
Reasons for inclusion and exclusion should be recorded
Step 3: Assessing the quality of studies
Study quality assessment is relevant to every step of a review.
Question formulation (Step 1) and study selection criteria (Step
2) should describe the minimum acceptable level of design.
Selected studies should be subjected to a more refined quality
assessment by use of general critical appraisal guides and
design-based quality checklists (Step 3). These detailed
quality assessments will be used for exploring heterogeneity
and informing decisions regarding suitability of meta-analysis
(Step 4). In addition they help in assessing the strength of
inferences and making recommendations for future research
(Step 5)
Step 4: Summarizing the evidence
Data synthesis consists of tabulation of study characteristics,
quality and effects as well as use of statistical methods for
exploring differences between studies and combining their
effects (meta-analysis). Exploration of heterogeneity and its
sources should be planned in advance (Step 3). If an overall
meta-analysis cannot be done, subgroup meta-analysis may
be feasible
Step 5: Interpreting the findings
The issues highlighted in each of the four steps above should
be met. The risk of publication bias and related biases should
be explored. Exploration for heterogeneity should help
determine whether the overall summary can be trusted, and, if
not, the effects observed in high-quality studies should be
used for generating inferences. Any recommendations should
be graded by reference to the strengths and weaknesses of
the evidence
Box 1 The steps in a systematic review
the question and the structure of the research design are
shown in Figure 1. This paper focuses only on the question
of safety related to the outcomes described below.
Free-form question
Is it safe to provide population-wide drinking water
fluoridation to prevent caries?
Structured question
. The populations—Populations receiving drinking water
sourced through a public water supply
. The interventions or exposures—Fluoridation of drinking
water (natural or artificial) compared with non-
fluoridated water
. The outcomes—Cancer is the main outcome of interest
for the debate in your health authority
. The study designs—Comparative studies of any design
examining the harmful outcomes in at least two
population groups, one with fluoridated drinking water
and the other without. Harmful outcomes can be rare
and they may develop over a long time. There are
considerable difficulties in designing and conducting
safety studies to capture these outcomes, since a large
number of people need to be observed over a long
period. These circumstances demand observational, not
randomized studies. With this background, systematic
reviews on safety have to include evidence from studies
with a range of designs.
STEP 2: IDENTIFY ING RELEVANT PUBLICATIONS
To capture as many relevant citations as possible, a wide
range of medical, environmental and scientific databases
were searched to identify primary studies of the effects of
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Figure 1 Structured questions for systematic reviews and relations between question components in a comparative study
water fluoridation. The electronic searches were supple-
mented by hand searching of Index Medicus and Excerpta
Medica back to 1945. Furthermore, various internet engines
were searched for web pages that might provide references.
This effort resulted in 3246 citations from which relevant
studies were selected for the review. Their potential
relevance was examined, and 2511 citations were excluded
as irrelevant. The full papers of the remaining 735 citations
were assessed to select those primary studies in man that
directly related to fluoride in drinking water supplies,
comparing at least two groups. These criteria excluded 481
studies and left 254 in the review. They came from thirty
countries, published in fourteen languages between 1939
and 2000. Of these studies 175 were relevant to the
question of safety, of which 26 used cancer as an outcome.
STEP 3: ASSESSING STUDY QUALI TY
Design threshold for study selection
Adequate study design as a marker of quality, is listed as an
inclusion criterion in Box 1. This approach is most
applicable when the main source of evidence is randomized
studies. However, randomized studies are almost impos-
sible to conduct at community level for a public health
intervention such as water fluoridation. Thus, systematic
reviews assessing the safety of such interventions have to
include evidence from a broader range of study designs.
Consideration of the type and amount of research likely to
be available led to inclusion of comparative studies of any
design. In this way, selected studies provided information
about the harmful effects of exposure to fluoridated water
compared with non-exposure.
Quality a ssessment of safety studies
After studies of an acceptable design have been selected,
their in-depth assessment for the risk of various biases
allows us to gauge the quality of the evidence in a more
refined way. Biases either exaggerate or underestimate the
‘true’ effect of an exposure. The objective of the included
studies was to compare groups exposed to fluoridated
drinking water and those without such exposure for rates of
undesirable outcomes, without bias. Safety studies should
ascertain exposures and outcomes in such a way that the risk
of misclassification is minimized. The exposure is likely to
be more accurately ascertained if the study was prospective
rather than retrospective and if it was started soon after
water fluoridation rather than later. The outcomes of those
developing cancer (and remaining free of cancer) are likely
to be more accurately ascertained if the follow-up was long
and if the assessment was blind to exposure status.
When examining how the effect of exposure on
outcome was established, reviewers assessed whether the
comparison groups were similar in all respects other than
their exposure to fluoridated water. This is because the
other differences may be related to the outcomes of interest
independent of the drinking-water fluoridation, and this
would bias the comparison. For example, if the people
exposed to fluoridated water had other risk factors that
made them more prone to have cancer, the apparent
association between exposure and outcome might be
explained by the more frequent occurrence of these factors
among the exposed group. The technical word for such
defects is confounding. In a randomized study, confounding
factors are expected to be roughly equally distributed
between groups. In observational studies their distribution
may be unequal. Primary researchers can statistically adjust
for these differences, when estimating the effect of
exposure on outcomes, by use of multivariable modelling.
Put simply, use of a prospective design, robust
ascertainment of exposure and outcomes, and control for
confounding are the generic issues one would look for in
quality assessment of studies on safety. Consequently,
studies may range from satisfactorily meeting quality
criteria, to having some deficiencies, to not meeting the
criteria at all, and they can be assigned to one of three
prespecified quality categories as shown in Table 1. A
quality hierarchy can then be developed, based on the
degree to which studies comply with the criteria. None of
the studies on cancer were in the high-quality category, but
this was because randomized studies were non-existent and
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Table 1 Description of quality assessment of studies on safety of public water fluoridation
Quality categori es High Moderate Low
Prospective design Prospective Prospective Prospective or retrospective
Ascertainment of exposure Study began within 1 year of
fluoridation
Study began within 3 years
of fluoridation
Study began 4 3 years after
fluoridation
Ascertainment of outcome Follow-up for at least 5 years
and blind assessment
Long follow-up and blind
assessment
Short follow-up and unblinded
assessment
Control for confounding Adjustment for at least three
confounding factors (or use
of randomization)
Adjustmentforatleastone
confounding factor
No adjustment for confounding
factors
control for confounding was not always ideal in the
observational studies. There were 8 studies of moderate
quality and 18 of low quality.
STEP 4: SUMMARIZING THE EVIDENCE
To summarize the evidence from studies of variable design
and quality is not easy. The original review
3
provides details
of how the differences between study results were
investigated and how they were summarized (with or
without meta-analysis). This paper restricts itself to
summarizing the findings narratively. The association
between exposure to fluoridated water and cancer in
general was examined in 26 studies. Of these, 10 examined
all-cause cancer incidence or mortality, in 22 analyses. Of
these, 11 analyses found a negative association (fewer
cancers due to exposure), 9 found a positive one and 2
found no association. Only 2 studies reported statistically
significant differences. Thus no clear association between
water fluoridation and increased cancer incidence or
mortality was apparent. Bone/joint and thyroid cancers
were of particular concern because of fluoride uptake by
these organs. Neither the 6 studies of osteosarcoma nor the
2 studies of thyroid cancer and water fluoridation revealed
significant differences. Overall no association was detected
between water fluoridation and mortality from any cancer.
These findings were also borne out in the moderate-quality
subgroup of studies.
STEP 5: INTERPRETING THE FINDINGS
In the fluoridation example, the focus was on the safety of a
community-based public health intervention. The generally
low quality of available studies means that the results must
be interpreted with caution. However, the elaborate efforts
in searching an unusually large number of databases provide
some safeguard against missing relevant studies. Thus the
evidence summarized in this review is likely to be as good as
it will get in the foreseeable future. Cancer was the harmful
outcome of most interest in this instance. No association
was found between exposure to fluoridated water and
specific cancers or all cancers. The interpretation of the
results may be generally limited because of the low quality
of studies, but the findings for the cancer outcomes are
supported by the moderate-quality studies.
RESOLUTION
After having spent some time reading and understanding the
review, you are impressed by the sheer amount of published
work relevant to the question of safety. However, you are
somewhat disappointed by the poor quality of the primary
studies. Of course, examination of safety only makes sense
in a context where the intervention has some beneficial
effect. Benefit and harm have to be compared to provide the
basis for decision making. On the issue of the beneficial
effect of public water fluoridation, the review
3
reassures
you that the health authority was correct in judging that
fluoridation of drinking water prevents caries. From the
review you also discovered that dental fluorosis (mottled
teeth) was related to concentration of fluoride. When the
interest groups raise the issue of safety again, you will be
able to declare that there is no evidence to link cancer with
drinking-water fluoridation; however, you will have to
come clean about the risk of dental fluorosis, which appears
to be dose dependent, and you may want to measure the
fluoride concentration in the water supply and share this
information with the interest groups.
The ability to quantify the safety concerns of your
population through a review, albeit from studies of
moderate to low quality, allows your health authority, the
politicians and the public to consider the balance between
beneficial and harmful effects of water fluoridation. Those
who see the prevention of caries as of primary importance
will favour fluoridation. Others, worried about the
disfigurement of mottled teeth, may prefer other means
of fluoride administration or even occasional treatment for
dental caries. Whatever the opinions on this matter, you are
able to reassure all parties that there is no evidence that
fluoridation of drinking water increases the risk of cancer.
CONCLUSION
With increasing focus on generating guidance and
recommendations for practice through systematic reviews,
healthcare professionals need to understand the principles of
preparing such reviews. Here we have provided a brief
step-by-step explanation of the principles. Our book
1
describes them in detail.
REFERENCES
1 Khan KS, Kunz R, Kleijnen J, Antes G. Systematic Reviews to Support
Evidence-Based Medicine. How to Review and Apply findings of Health Care
Research. London: RSM Press, 2003
2 McDonagh M, Whiting P, Bradley M, et al. Systematic review of water
fluoridation. BMJ 2000;321:855–9
3 NHS Centre for Reviews and Dissemination (CRD). A systematic
review of water fluoridation. CRD Report 18. York: University of York,
2000 [http://www.york.ac.uk/inst/crd/fluorid.htm]
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