Citation searching uses seed references (usually the set of included records from the results of the database search) and checks whether additional relevant articles can be found through forward citation searching and backward citation searching.
You may like to refer to the Guidance on terminology, application, and reporting of citation searching: the TARCiS statement. It provides guidance for systematic reviewers on when and how to conduct citation searching and on how to report it.
Forward citation searching looks forward in the literature. It takes relevant articles (seed articles) and looks for newer articles that have cited them. It is also known as Forward snowballing or Cited by searching.
Useful databases for forward snowballing are:
These automation tools help generate all forwards and backwards citations (one level deep):
Note! You need to report on the use of automation tools as per the PRISMA guidelines.
Backward citation searching looks backwards in the literature. It takes relevant articles and looks at their reference lists for other articles that might be relevant. This technique is also called Backward snowballing or Reference searching.
Useful databases for reference searching are:
If reference lists are reviewed manually it is called Reference list checking.
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.
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.)
