Enough evidence and other endings: a descriptive study of stable Cochrane systematic reviews in 2019.

BackgroundFrom 2006 to 2019, Cochrane reviews could be designated "stable" if they were not being updated but highly likely to be current. This provides an opportunity to observe practice in ending systematic reviewing and what is regarded as enough evidence. MethodsWe identified Cochrane reviews designated stable in 2013 and 2019 and reasons for this designation. For those with conclusions stated to be so firm that new evidence is unlikely to change them, we assessed conclusions, strength of evidence ratings, and recommendations for further research. We assessed the fate of the 2013 stable reviews. We also estimated usage of formal analytic methods to determine when there is enough evidence in protocols for Cochrane reviews. ResultsCochrane reviews were rarely designated stable. In 2019, there were 507 stable Cochrane reviews (6.6% of 7,645 non-withdrawn reviews). The most common reasons related to no, little, or infrequent research activity expected (331 of 505; 65.5%). Only 39 reviews were stable because of firm conclusions unlikely to be changed by new evidence (7.7%), but that declaration was mostly not supported by judgments made in the review about strength of evidence and implications for research. Among the 180 reviews stable in 2013, 16 reverted to normal status (8.9%), with 2 of those changing conclusions because of new studies. Few Cochrane protocols specified an analytic method for determining when there was enough evidence to stop updating the review (116 of 2,415; 4.8%). ConclusionCochrane reviews were more likely to end because important future primary research activity was believed to be unlikely, than because there was enough evidence. Judgments about the strength of evidence and need for research were often inconsistent with the declaration that conclusions were unlikely to change. The inconsistencies underscore the need for reliable analytic methods to support decision-making about the conclusiveness of evidence.