BMC Medical Research Methodology 2008, 8:45
http://www.biomedcentral.com/1471-2288/8/45
Background
The systematic review is an important technology for the
evidence-informed policy and practice movement, which
aims to bring research closer to decision-making [1,2].
This type of review uses rigorous and explicit methods to
bring together the results of primary research in order to
provide reliable answers to particular questions [3-6]. The
picture that is presented aims to be distorted neither by
biases in the review process nor by biases in the primary
research which the review contains [7-10]. Systematic
review methods are well-developed for certain types of
research, such as randomised controlled trials (RCTs).
Methods for reviewing qualitative research in a systematic
way are still emerging, and there is much ongoing devel-
opment and debate [11-14].
In this paper we present one approach to the synthesis of
findings of qualitative research, which we have called 'the-
matic synthesis'. We have developed and applied these
methods within several systematic reviews that address
questions about people's perspectives and experiences
[15-18]. The context for this methodological develop-
ment is a programme of work in health promotion and
public health (HP & PH), mostly funded by the English
Department of Health, at the EPPI-Centre, in the Social
Science Research Unit at the Institute of Education, Uni-
versity of London in the UK. Early systematic reviews at
the EPPI-Centre addressed the question 'what works?' and
contained research testing the effects of interventions.
However, policy makers and other review users also posed
questions about intervention need, appropriateness and
acceptability, and factors influencing intervention imple-
mentation. To address these questions, our reviews began
to include a wider range of research, including research
often described as 'qualitative'. We began to focus, in par-
ticular, on research that aimed to understand the health
issue in question from the experiences and point of view
of the groups of people targeted by HP&PH interventions
(We use the term 'qualitative' research cautiously because
it encompasses a multitude of research methods at the
same time as an assumed range of epistemological posi-
tions. In practice it is often difficult to classify research as
being either 'qualitative' or 'quantitative' as much research
contains aspects of both [19-22]. Because the term is in
common use, however, we will employ it in this paper).
When we started the work for our first series of reviews
which included qualitative research in 1999 [23-26], there
was very little published material that described methods
for synthesising this type of research. We therefore experi-
mented with a variety of techniques borrowed from stand-
ard systematic review methods and methods for analysing
primary qualitative research [15]. In later reviews, we were
able to refine these methods and began to apply thematic
analysis in a more explicit way. The methods for thematic
synthesis described in this paper have so far been used
explicitly in three systematic reviews [16-18].
The review used as an example in this paper
To illustrate the steps involved in a thematic synthesis we
draw on a review of the barriers to, and facilitators of,
healthy eating amongst children aged four to 10 years old
[17]. The review was commissioned by the Department of
Health, England to inform policy about how to encourage
children to eat healthily in the light of recent surveys high-
lighting that British children are eating less than half the
recommended five portions of fruit and vegetables per
day. While we focus on the aspects of the review that relate
to qualitative studies, the review was broader than this
and combined answering traditional questions of effec-
tiveness, through reviewing controlled trials, with ques-
tions relating to children's views of healthy eating, which
were answered using qualitative studies. The qualitative
studies were synthesised using 'thematic synthesis' - the
subject of this paper. We compared the effectiveness of
interventions which appeared to be in line with recom-
mendations from the thematic synthesis with those that
did not. This enabled us to see whether the understand-
ings we had gained from the children's views helped us to
explain differences in the effectiveness of different inter-
ventions: the thematic synthesis had enabled us to gener-
ate hypotheses which could be tested against the findings
of the quantitative studies - hypotheses that we could not
have generated without the thematic synthesis. The meth-
ods of this part of the review are published in Thomas et
al. [27] and are discussed further in Harden and Thomas
[21].
Qualitative research and systematic reviews
The act of seeking to synthesise qualitative research means
stepping into more complex and contested territory than
is the case when only RCTs are included in a review. First,
methods are much less developed in this area, with fewer
completed reviews available from which to learn, and sec-
ond, the whole enterprise of synthesising qualitative
research is itself hotly debated. Qualitative research, it is
often proposed, is not generalisable and is specific to a
particular context, time and group of participants. Thus,
in bringing such research together, reviewers are open to
the charge that they de-contextualise findings and
wrongly assume that these are commensurable [11,13].
These are serious concerns which it is not the purpose of
this paper to contest. We note, however, that a strong case
has been made for qualitative research to be valued for the
potential it has to inform policy and practice [11,28-30].
In our experience, users of reviews are interested in the
answers that only qualitative research can provide, but are
not able to handle the deluge of data that would result if
they tried to locate, read and interpret all the relevant
research themselves. Thus, if we acknowledge the unique
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