Defining the research area

The first step in conducting a meta-analysis implies defining very clearly the focus of the meta-analysis. It starts from nothing more than a general area of interest, and its purpose is to turn that area into a precise, answerable research question: what is the research question addressed by the meta-analytic review? For instance, we may want to study the efficacy of an intervention aimed at reducing hypertension, or gender differences in depression, or associations between physical exercise and body weight. More generally, in the meta-analysis we can compare differences between two groups or examine associations between two variables.

The research question should be rooted in a clear theoretical background. This keeps the review from turning into an open-ended "fishing" exercise and from mixing conceptually different constructs under the same effect size — a mistake often summed up as mixing apples and oranges.

A meta-analytic review earns its place over a narrative review at this stage already: framing the question around an effect size to be estimated, rather than a topic to be summarized, is what makes the rest of the process — transparent study selection, an explicit weight for each study, replicable conclusions — possible in the first place. It also avoids the trap of the older vote-counting method, which just tallies how many studies found a significant result versus a null one and ignores the size of the effect entirely.

The result of this step is a clearly stated research question, framed around an effect size, that anchors every step that follows — starting with the inclusion and exclusion criteria.