Citation tracking uses an identified relevant article and checks whether additional relevant articles can be found through forward snowballing (cited by searching) and/or backward snowballing (reference searching).
Forward snowballing looks forward in the literature. It takes relevant articles and looks for newer articles that have cited them.
Useful Databases for forward snowballing are:
Backward snowballing looks backwards in the literature. It takes relevant articles and looks at their reference lists for other articles that might be relevant.
It can be fruitful to search the reference lists of relevant existing systematic reviews and meta-analyses, identified studies (included and excluded studies) or other related articles.
Useful Databases for reference searching are:
According to Cochrane, searching reference lists is mandatory; reference lists in included studies and any identified relevant systematic reviews should be checked (Cochrane Handbook, 4.3.5.)
Since investigators may selectively cite studies with positive results, reference lists should be used with caution as an adjunct to other search methods.