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Summary reading material Systematic reviews for Health & Society
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Summary readingmaterial HSO31306
Petticrew, M., & Roberts, H.
(2006). Systematic Reviews in
the Social Sciences: A Practical
Guide.
,WEEK 1
Chapter 1 Why do we need systematic reviews?
1.1 Introduction
Systematic literature reviews are a method of making sense of large bodies of information, and a
means of contributing to the answers to questions about what works and what does not – and many
other types of question too. They are a method of mapping out areas of uncertainty, and identifying
where little or no relevant research has been done, but where new studies are needed. Systematic
reviews also flag up areas where spurious certainty abounds. These are areas where we think we
know more than we do, but where in reality there is little convincing evidence to support our beliefs.
There are few studies that are so methodologically sound, whose results are so generalizeable and
that leave us so certain that the results represent a good approximation of the ‘‘truth,’’ that we
should accept their findings outright.
One problem with interpreting and using research is that the research behind the headlines is often
so far removed from real-life settings that it may be difficult for policymakers or the to know whether
the results are to be taken seriously. Studies produce such conflicting findings for reasons other than
methodological shortcomings, or authorial bias. There may be simple statistical reasons why the
results of studies vary; 20 studies of the same intervention will produce a range of estimates of its
impact, and one of these studies will, by chance alone, produce statistically significant findings even
if the intervention is ineffective. Moreover the findings may often be statistically significant, but not
socially significant. If we find studies with contradictory answers we might not know which study to
believe, and perhaps the most sensible approach might be to rely on a literature review written by an
expert in the field. However, literature reviews may themselves be biased, and by carefully selecting
which studies to review, it is possible to produce two similar reviews that come to entirely opposite
conclusions.
1.2 Bias in traditional reviews
Most scientists operate on a double standard; they go to great lengths to define the methods they
used to minimise biases and random errors in their reports on the results of new research, but they
often do not attempt to apply scientific principles in their discussions of how the newly generated
evidence accords with previously available information. It is a fair criticism of scientific research that
it is often difficult to know which, if any, study to believe. The traditional scientific approach to this
problem is to carry out a literature review. This is often conducted by an expert or other well-known
figure in the field, but general expertise and high profile may be poor indicators of the ability to
produce an unbiased and reliable summary of the evidence. Literature reviews, even those written
by experts, can be made to tell any story one wants them to, and failure by literature reviewers to
apply scientific principles to the process of reviewing the evidence, just as one would to primary
research, can lead to biased conclusions, and to harm and wasted resources. As we shall see in later
chapters, researchers can also be influenced, consciously or otherwise, by their own pet theories, by
their funders, and sometimes by the perceived need to produce positive findings in order to get
published. All these make it difficult to work out where the balance of truth lies in many areas of
science. Of course many traditional literature reviews are excellent. Moreover, not every review
needs to be a ‘‘systematic’’ review. However, if a review purports to be an authoritative summary of
what ‘‘the evidence’’ says, then the reader is entitled to demand that this is a comprehensive,
objective, and reliable overview, and not a partial review of a convenience sample of the author’s
,favorite studies. One way of giving the reader this assurance is by describing the methods used (as
for any study).
1.3 Information overload
The problem is not just one of inconsistency, but one of information overload. The past 20 years have
seen an explosion in the amount of research information available to decision makers and social
researchers alike. As well as a huge increase in the publication of science papers, the number of web-
based sources of apparently relevant information has increased dramatically. In many ways, this is a
welcome democratization of knowledge, or at least a means by which information can be made
much more widely available.
1.4 The role of systematic literature reviews
As noted above, those who use research information, among whom we can number citizens,
researchers, and people making decisions about the delivery or organization of services, rely on
reviews to help organize and prioritize the most relevant information. Alongside ‘‘traditional’’ or
‘‘narrative’’ reviews, a new type of review has appeared in recent years: the systematic literature
review. Systematic reviews are literature reviews that adhere closely to a set of scientific methods
that explicitly aim to limit systematic error (bias), mainly by attempting to identify, appraise and
synthesize all relevant studies (of whatever design) in order to answer a particular question (or set of
questions). In carrying out this task they set out their methods in advance, and in detail, as one
would for any piece of social research. In this respect, they are quite unlike most ‘‘traditional’’
narrative reviews.
The systematic review is more ‘‘fit for the purpose’’ of answering specific questions and testing
hypotheses than the traditional review. It is less of a discussion of the literature, and more of a
scientific tool; but it can also do more than this, and can be used to summarize, appraise, and
communicate the results and implications of otherwise unmanageable quantities of research.
Systematic and non-systematic reviews fulfill very different needs. Non-systematic reviews can be
reviews that provide valuable summaries over a wide topic area, and which make no pretence to do
anything other than present an overview. They may also use a literature review to present an
argument. The systematic review by contrast adopts a particular methodology in an endeavor to
limit bias, with the overall aim of producing a scientific summary of the evidence in any area. In this
respect, systematic reviews are simply another research method, and in many respects they are very
similar to a survey – though in this case they involve a survey of the literature, not of people. Perhaps
most importantly systematic literature reviews usually aim to answer a specific question or test a
specific hypothesis, rather than simply summarizing ‘‘all there is to know’’ about a particular issue.
We emphasize however that in answering questions about effectiveness or causation, a well-
conducted systematic review should be considered the most authoritative source of information.
However, broader questions may require the use of other tools from the social scientist’s toolbox,
and good traditional reviews play an important role.
1.5 When a systematic review is of value
Systematic reviews are particularly valuable as a means of reviewing all the evidence on a
particular question if there is some uncertainty about the answer. If it is unclear whether a
, particular intervention is effective, then a systematic review of the available evidence may help
resolve the issue. The influence of systematic reviews has grown rapidly as potential users have
become aware that they provide a means of dealing with the information mountain, by allowing
large amounts of research information to be distilled into a manageable form. A further attraction to
users lies in the potential to allow decisions to be made on a transparent and potentially defendable
basis, as it draws on all relevant scientifically sound research, rather than on single studies.
Systematic reviews to inform policy
Politicians and policymakers are increasingly interested in evidence-based decision making. They are
under pressure to look to research for solutions to policy problems, and to justify programs by
reference to the knowledge base. For policymakers, systematic reviews may provide robust, reliable
summaries of the most reliable evidence: a valuable backdrop of evidence on which decisions about
policies can draw.
Systematic reviews to support practice
Research has long played a part in informing the practice of those involved in the delivery of social
policies, although a part which is sometimes more extensive than practitioners, and less than
researchers would like. The growth of interest in evidence-based practice has increased pressure on
practitioners to demonstrate that their existing work and their new practices are based on the best
available research evidence. Systematic reviews therefore provide a key source of evidence-based
information to support and develop practice as well as to support professional development.
1.6 Why do a systematic review? A rationale
Single studies can usefully be seen as similar to single respondents in a survey. The results from one
respondent may say something, and sometimes something very important, but one might well get
the opposite answer from the next respondent. It is more likely that one will learn more by
examining data from other respondents, by looking at the range of answers and examining why
those answers vary, and by attempting to summarize them.
1.7 An unsystematic history of systematic reviews
The idea of systematically reviewing research has been around for many decades. From the 1930s
onwards, and possibly even before, the specific term ‘‘systematic review’’ was being used to refer to
literature reviews. Neither the term ‘‘systematic review’’ nor the general approach of systematic
literature reviewing are particularly new, nor particularly biomedical.
1.8 The Cochrane and Campbell Collaborations: International
collaborations of systematic reviewers
By the 1980s systematic reviews in health care in particular began to be published in greater
numbers, and in the UK in the early 1990s two centers were established to prepare and maintain
systematic reviews of the effects of health care interventions. The goals of the Cochrane
Collaboration have been adopted more recently by the Campbell Collaboration, which aims to
prepare, maintain, and disseminate the results of systematic reviews.
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