Background: Inadequate planning, selective reporting, and incomplete reporting of outcomes in randomized controlled trials (RCTs) contribute to the problem of waste of research. We aimed to describe such a waste and to examine to what extent this waste could be avoided.
Methods: This research-on-research study was based on RCTs included in Cochrane reviews with a summary of findings (SoF) table. We considered the outcomes reported in the SoF tables as surrogates for important outcomes for patients and other decision makers. We used a three-step approach. (1) First, in each review, we identified, for each important outcome, RCTs that were excluded from the corresponding meta-analysis. (2) Then, for these RCTs, we systematically searched for registrations and protocols to distinguish between inadequate planning (an important outcome was not reported in registries or protocols), selective reporting (an important outcome was reported in registries or protocols but not in publications), and incomplete reporting (an important outcome was incompletely reported in publications). (3) Finally, we assessed, with the consensus of five experts, the feasibility and cost of measuring the important outcomes that were not planned. We considered inadequately planned or selectively or incompletely reported important outcomes as avoidable waste if the outcome could have been easily measured at no additional cost based on expert evaluation.
Results: Of the 2711 RCTs included in the main comparison of 290 reviews, 2115 (78%) were excluded from at least one meta-analysis of important outcomes. Every trial contributed to 55%, on average, of the meta-analyses of important outcomes. Of the 310 RCTs published in 2010 or later, 156 were registered. Inadequate planning affected 79% of these RCTs, whereas incomplete and selective reporting affected 41% and 15%, respectively. For 63% of RCTs, we found at least one missing important outcome for which the waste was avoidable and for 30%, the waste was avoidable for all important outcomes.
Conclusions: Most of the RCTs included in our sample did not contribute to all the important outcomes in meta-analyses, mostly because of inadequate planning or incomplete reporting. A large part of this waste of research seemed to be avoidable.
Keywords: Core outcome set; Outcome; Randomized controlled trial; Selective reporting; Waste of research.