Defining inclusion and exclusion criteria

Once the research question is set (see Defining the research area), the next step is to translate it into inclusion and exclusion criteria: explicit rules for which studies are eligible or not for being included in the meta-analysis. They refer to:

• characteristics of the study (e.g., population, design)
• characteristics of the publication (e.g., language, type, year of publication)

Restricting inclusion to peer-reviewed, English-language journals is a common choice, but it risks overestimating the effect under study: unpublished and non-English studies (the "gray literature") tend to differ systematically from published ones, because publication itself is driven partly by whether results are statistically significant — see Evaluating publication bias. Including gray literature reduces this risk, at the cost of a more laborious search.

The publication-year criterion deserves the same explicit justification as any other: searching without a start date casts the widest net, but restricting it usually only makes sense when there's a clear starting point for the literature (e.g., the year a foundational theory or measure was introduced) or when the review is explicitly updating an earlier one — in which case the criterion is simply "published after the previous review's search date."

The result of this step is an explicit, written set of eligibility criteria, decided before the search begins, that will be applied consistently when searching and selecting studies.