Study flow diagrams are used to illustrate the results of the search and the process of screening and selecting studies for inclusion in the review. Figure 11.2.a provides an example of a flow diagram, following the template described in the PRISMA statement (Liberati 2009). A flow diagram using the PRISMA template may be created within RevMan, and RevMan also includes the facility to create a flow diagram with a flexible structure.
It is important that the text of the flow diagram draws a clear distinction between studies and records. Records are information sources about a study, such as journal articles, book chapters, web pages and other documents. Studies are the research enterprises themselves, typically randomized trials in the context of a Cochrane review. Usually a flow diagram will start by describing numbers of records retrieved (the majority of which will typically be from bibliographic databases). Following de-duplication, the records will have been mapped onto distinct studies (see Chapter 7, Section 7.2), and the flow diagram will reflect this by switching its emphasis to studies.
The flow diagram should present:
number of unique records identified by the searches;
number of records excluded after preliminary screening (e.g. of titles and abstracts);
number of records retrieved in full text;
number of records or studies excluded after assessment of the full text, with brief reasons;
number of studies meeting eligibility criteria for the review (and thus contributing to qualitative synthesis); and
number of studies contributing to the main outcome.
Further guidance on preparing a study flow diagram is provided in the explanation and elaboration paper for the PRISMA statement (Liberati 2009) and in a review of published flow diagrams (Hind 2007).