Meta-analysis - Inclusion and Exclusion for Studies

Once a research question for meta-analysis has been formulated, the next step is to establish inclusion and exclusion criteria.

Research question itself guides the inclusion and exclusion criteria. It excludes studies that don't fit its four components, i.e. study population (e.g. patients with CHD), exposure (e.g. an intervention vs no intervention), control population (e.g. patients without CHD), and clinical outcome (e.g. death). Only those studies that address these aspects can be considered for inclusion.

There are other criteria that are often used:

STUDY DESIGN: e.g. randomized placebo-controlled trials, pre-post design or repeated-measure design studies, observational studies etc.

LANGUAGE: published in English only, or published in English and other languages.

PUBLICATION TYPE: e.g. articles published in peer-review journals, presented at conferences, postgraduate thesis, unpublished data, etc.

KEY VARIABLES: Only studies that give information about the key variables under study can be included. This information should be enough to calculate effect size (will discuss some other time).

TIME FRAME: a particular start date, such as 1966, to include only modern studies.

NUMBER OF SUBJECTS IN THE STUDY: Some have suggested including studies only with a large number of subjects, however, this is not generally accepted. This approach has not been evaluated fully.

DURATION OF THE STUDY: in other words length of follow-up, such as at least 12 months of follow-up etc. This is important if one is interested in the long-term effects of a particular exposure (intervention).