Searching the literature

With the inclusion and exclusion criteria in place, the next step is to retrieve every study that could plausibly satisfy them. A comprehensive search combines multiple strategies rather than relying on a single one:

• electronic reference databases (e.g., PsycINFO, ERIC, MEDLINE, EMBASE, Scopus, Web of Science)
• reference lists of reviews and pertinent primary studies already available on the topic
• indexes of journals that publish most of the papers on the topic, useful for catching articles ahead of print
• direct contact with experts in the field

A keyword search alone tends to miss relevant work, so it's usually combined with a citation search — finding every paper that cites a foundational article on the topic — and with a check of recent and ahead-of-print issues of the journals most likely to publish on it, which catches papers not yet indexed in the databases.

When gray literature (unpublished or hard-to-find studies) is included, these strategies are supplemented with searches in dissertation-abstract databases and conference programs, and outreach to authors and scientific associations — more thorough, but more time-consuming and harder to replicate exactly.

The result of this step is a pool of retrieved references — including duplicates and clearly ineligible records at this stage — to be screened in Selecting primary studies. Results of this step feed the identification stage of a PRISMA flow diagram.