The epidemiology of systematic review updates: a longitudinal study of updating of Cochrane reviews, 2003 to 2018.

BackgroundThe Cochrane Collaboration has been publishing systematic reviews in the Cochrane Database of Systematic Reviews (CDSR) since 1995, with the intention that these be updated periodically. ObjectivesTo chart the long-term updating history of a cohort of Cochrane reviews and the impact on the number of included studies. MethodsThe status of a cohort of Cochrane reviews updated in 2003 was assessed at three time points: 2003, 2011, and 2018. We assessed their subject scope, compiled their publication history using PubMed and CDSR, and compared them to all Cochrane reviews available in 2002 and 2017/18. ResultsOf the 1,532 Cochrane reviews available in 2002, 11.3% were updated in 2003, with 16.6% not updated between 2003 and 2011. The reviews updated in 2003 were not markedly different to other reviews available in 2002, but more were retracted or declared stable by 2011 (13.3% versus 6.3%). The 2003 update led to a major change of the conclusions of 2.8% of updated reviews (n = 177). The cohort had a median time since publication of the first full version of the review of 18 years and a median of three updates by 2018 (range 1-11). The median time to update was three years (range 0-14 years). By the end of 2018, the median time since the last update was seven years (range 0-15). The median number of included studies rose from eight in the version of the review before the 2003 update, to 10 in that update and 14 in 2018 (range 0-347). ConclusionsMost Cochrane reviews get updated, however they are becoming more out-of-date over time. Updates have resulted in an overall rise in the number of included studies, although they only rarely lead to major changes in conclusion.