Background: Publication and outcome reporting bias are well known problems when conducting a systematic review. One way to attempt to overcome these problems is to search for unpublished studies or data. The Cochrane Handbook recommends searching beyond the published article by contacting experts or authors and by searching conference abstracts, the grey literature and trial registries. As one of the most update and freely available guides to systematic reviews the Cochrane Handbook is used by both Cochrane and non-Cochrane reviewers.
Objectives: We sought to identify the proportion of systematic reviews of adverse effects that search for unpublished data, and the success rates of identifying unpublished data for inclusion in a systematic review.
Methods: Two reviewers independently screened all records published in 2014 in the Database of Abstracts of Reviews of Effects (DARE) for systematic reviews where the primary aim was to evaluate an adverse effect or effects. Data were extracted on the types of adverse effects and interventions evaluated, sources searched, how many unpublished studies were included and type of unpublished data included.
Results: From 9129 DARE abstracts, 348 met our inclusion criteria. Most reviews evaluated a drug intervention (237/348, 68%) with specified adverse effects (250/348, 72%). Over a third (136/348, 39%) searched a specific source for unpublished data, such as conference abstracts or trial registries. However, less than half of these reviews (62/136, 46%) included unpublished data in their review. The most popular sources searched were conference abstracts, contacting authors and ClinicalTrials.gov. Overall over a fifth of all the reviews included some unpublished data (78/348, 22%). Although most of these reviews searched specific sources of unpublished data (62/78, 79%), others did not, but included sources that contain unpublished studies in addition to published studies (such as Embase or the Cochrane Central Register of Controlled Trials (CENTRAL)) (16/78, 11%).
Conclusions: Most reviews of adverse effects do not search specifically for unpublished data and less than half of those that do are successful.