Background: The current best evidence is usually informed by systematic reviews (SRs) of randomized controlled trials (RCTs). The pooled treatment effect estimates of meta-analyses can be biased when RCTs are omitted from the meta-analyses because of outcome-related reasons (i.e., missing evidence).
Objectives: We aimed to assess the prevalence of RCTs omitted from meta-analysis of SRs in the rehabilitation field and the reasons for missing evidence.
Methods: This is a cross-sectional meta-research study, prospectively registered (https://osf.io/p25zy/). We started from a sample of 827 SRs identified by Gianola et al. We defined the index meta-analysis (IMA) as the meta-analysis of the primary outcome of the main comparison or, if not clearly identified, the first one reported in the results section. Starting from all studies eligible for the chosen comparison, we identified all RCTs omitted (i.e., not included) from the IMA. The reasons for RCT omission were assessed using and adapting the classification of Yordanov et al (Table 1).
Results: Starting from a cohort of 827 SRs, 131 IMAs comprising a total of 1,761 eligible RCTs were selected. Out of all eligible studies, 717 RCTs (40,7%) were omitted from 100 IMAs: for 38.8% the reason was not clear due to lack of a registered protocol (i.e., “Unable to distinguish between selective reporting and inadequate planning”), 17% due to “inadequate or alternative or missing planning”, 15,1% were justified not to be included, 8,4% due to “incomplete reporting”, 5,2% due to other situations, 3,3% due to “selective reporting”, and 11,7% were not assessed being non-English trials or trials with full text not available (Table 2).
Conclusions: Outcome non-reporting bias (i.e., “selective reporting” and “incomplete reporting”) and lack of guidance on the outcomes to be measured as a minimum (i.e., “inadequate or alternative or missing planning”) represent important reasons for missing evidence. Similarly, lack of trial registration prevents the identification of the reason for missing evidence. Consequently, better reporting, prospective protocol registration, and the definition of core outcome sets for every clinical condition are needed to avoid research waste and improve conducting of RCTs in the rehabilitation field. Patient, public, and/or healthcare consumer involvement: Not applicable.

Table 1.pdf
Table 2.pdf