However, to increase confidence in their decisions, it is better to look at all of the available research. Single studies on a topic do provide some information. Meta-regression may best be used for this purpose, although it is not implemented in RevMan (see Section 9.6.4 ).When making decisions that affect many people, policy-makers, clinicians and other decision-makers may turn to research to help inform their choices.
#DEFINE META ANALYSIS IN PSYCHOLOGY SERIES#
It is legitimate for a systematic review to focus on examining the relationship between some clinical characteristic(s) of the studies and the size of intervention effect, rather than on obtaining a summary effect estimate across a series of studies (see Section 9.6 ).
Where possible these investigations should be specified a priori, i.e. There may be specific interest in a review in investigating how clinical and methodological aspects of studies relate to their results. For example, a meta-analysis may reasonably evaluate the average effect of a class of drugs by combining results from trials where each evaluates the effect of a different drug from the class. This is true if apples and oranges are of intrinsic interest on their own, but may not be if they are used to contribute to a wider question about fruit. A common analogy is that systematic reviews bring together apples and oranges, and that combining these can yield a meaningless result. It is often appropriate to take a broader perspective in a meta-analysis than in a single clinical trial. Meta-analysis should only be considered when a group of studies is sufficiently homogeneous in terms of participants, interventions and outcomes to provide a meaningful summary. Studies of each intervention should be analysed and presented separately. Sometimes a review will include studies addressing a variety of questions, for example when several different interventions for the same condition are of interest (see also Chapter 5, Section 5.6). The scope of a review will largely determine the extent to which studies included in a review are diverse. Empirical evidence suggests that some aspects of design can affect the result of clinical trials, although this is not always the case. In particular, heterogeneity associated solely with methodological diversity would indicate the studies suffer from different degrees of bias. Significant statistical heterogeneity arising from methodological diversity or differences in outcome assessments suggests that the studies are not all estimating the same quantity, but does not necessarily suggest that the true intervention effect varies. In other words, the true intervention effect will be different in different studies.ĭifferences between studies in terms of methodological factors, such as use of blinding and concealment of allocation, or if there are differences between studies in the way the outcomes are defined and measured, may be expected to lead to differences in the observed intervention effects. We will follow convention and refer to statistical heterogeneity simply as heterogeneity.Ĭlinical variation will lead to heterogeneity if the intervention effect is affected by the factors that vary across studies most obviously, the specific interventions or patient characteristics. Statistical heterogeneity manifests itself in the observed intervention effects being more different from each other than one would expect due to random error (chance) alone. Variability in the intervention effects being evaluated in the different studies is known as statistical heterogeneity, and is a consequence of clinical or methodological diversity, or both, among the studies. Variability in the participants, interventions and outcomes studied may be described as clinical diversity (sometimes called clinical heterogeneity), and variability in study design and risk of bias may be described as methodological diversity (sometimes called methodological heterogeneity). It can be helpful to distinguish between different types of heterogeneity.
Any kind of variability among studies in a systematic review may be termed heterogeneity. Inevitably, studies brought together in a systematic review will differ. For the current version, please go to /handbook/current or search for this chapter here. This is an archived version of the Handbook.
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