Checking reference lists to find additional studies for systematic reviews.

BACKGROUND Checking reference lists to identify relevant studies for systematic reviews is frequently recommended by systematic review manuals and is often undertaken by review authors. To date, no systematic review has explicitly examined the effectiveness of checking reference lists as a method to supplement electronic searching. OBJECTIVES To investigate the effectiveness of checking reference lists for the identification of additional, relevant studies for systematic reviews. Effectiveness is defined as the proportion of relevant studies identified by review authors solely by checking reference lists. SEARCH STRATEGY We searched the databases of The Cochrane Library (Issue 3, 2008), Library and Information Science abstracts (LISA) (1969 to July 2008) and MEDLINE (1966 to July 2008). We contacted experts in systematic review methods and examined reference lists of articles. SELECTION CRITERIA Studies of any design which examined checking reference lists as a search method for systematic reviews in any area. The primary outcome was the additional yield of relevant studies (i.e. studies not found through any other search methodologies); other outcomes were publication types identified and data pertaining to the costs (e.g. cost-effectiveness, cost-efficiency) of checking reference lists. DATA COLLECTION AND ANALYSIS We summarized data descriptively. MAIN RESULTS We included 12 studies (in 13 publications) in this review, but interpretability and generalizability of these studies is difficult and the study designs used were at high risk of bias. The additional yield (calculated by dividing the additional 'unique' yield identified by checking reference lists by the total number of studies found to be eligible within the study) of relevant studies identified through checking reference lists ranged from 2.5% to 42.7%. Only two studies reported yield information by publication type (dissertations and systematic reviews). No cost data were reported although one study commented that it was impossible to isolate the time spent on reference tracking since this was done in parallel with the critical appraisal of each paper, and for that particular study costs were not specifically estimated. AUTHORS' CONCLUSIONS There is some evidence to support the use of checking reference lists for locating studies in systematic reviews. However, this evidence is derived from weak study designs. In situations where the identification of all relevant studies through handsearching and database searching is difficult, it would seem prudent that authors of reviews check reference lists to supplement their searching. The challenge, therefore, is for review authors to recognize those situations.