Table 1 Definitions of commonly used methods that could recover falsely excluded studies

Reference list checking (backward citation tracking)

Checking the reference lists of the included studies and any relevant systematic reviews identified [7, 8]

Similarity searches (i.e., related articles)

Using a key article to identify additional relevant articles by using a “similar articles” option available in some databases and search engines (e.g., PubMed, Google Scholar) [7, 8]

Forward citation tracking of included studies

Using citation indexes for forward citation searching based on a key article [8]

Academic search engines

Keyword searches using an academic search engine (e.g., Google Scholar) [8]

Contacting experts/researchers/companies/other stakeholders

Contacting individuals and organizations for information about relevant studies [8]