BMC Med Res Methodol. 2023 Oct 21;23(1):245. doi: 10.1186/s12874-023-02064-7.
BACKGROUND: Systematic reviews of randomized controlled trials are the best evidence for informing on intervention effectiveness. Their results, however, can be biased due to omitted evidence in the quantitative analyses. We aimed to assess the proportion of randomized controlled trials omitted from meta-analyses in the rehabilitation field and explore related reasons.
METHODS: This is a cross-sectional meta-research study. For each systematic review included in a published selected sample in the rehabilitation field, we identified an index meta-analysis on the primary outcome and the main comparison. We then looked at all the studies considered eligible for the chosen comparison in the systematic review and identified those trials that have been omitted (i.e., not included) from each index meta-analysis. Reasons for omission were collected based on an eight-reason classification. We used descriptive statistics to describe the proportion of omitted trials overall and according to each reason.
RESULTS: Starting from a cohort of 827 systematic reviews, 131 index meta-analyses comprising a total of 1761 eligible trials were selected. Only 16 index meta-analyses included all eligible studies while 15 omitted studies without providing references. From the remaining 100 index meta-analyses, 717 trials (40,7%) were omitted overall. Specific reasons for omission were: “unable to distinguish between selective reporting and inadequate planning” (39,3%, N = 282), “inadequate planning” (17%, N = 122), “justified to be not included” (15,1%, N = 108), “incomplete reporting” (8,4%, N = 60), “selective reporting” (3,3%, N = 24) and other situations (e.g., outcome present but no motivation for omission) (5,2%, N = 37). The 11,7% (N = 84) of omitted trials were not assessed due to non-English language or full text not available.
CONCLUSIONS: Almost half of the eligible trials were omitted from their index meta-analyses. Better reporting, protocol registration, definition and adoption of core outcome sets are needed to prevent omission of evidence in systematic reviews.