Assessing the risk of performance and detection bias in Cochrane reviews as a joint domain is less accurate compared to two separate domains

Background Initially, the Cochrane risk of bias (RoB) tool had a domain for "blinding of participants, personnel and outcome assessors". In the 2011 tool, the assessment of blinding was split into two domains: blinding of participants and personnel (performance bias) and blinding of outcome assessors (detection bias). The aims of this study were twofold; first, to analyze the frequency of usage of the joint blinding domain (a single domain for performance and detection bias), and second, to assess the proportion of adequate assessments made in the joint versus single RoB domains for blinding by comparing whether authors' RoB judgments were supported by explanatory comments in line with the Cochrane Handbook recommendations. Methods We extracted information about the assessment of blinding from RoB tables (judgment, comment, and whether it was specified which outcome type; e.g., objective, subjective) of 729 Cochrane reviews published in 2015-2016. In the Cochrane RoB tool, judgment (low, unclear or high risk) needs to be accompanied by a transparent comment, in which authors provide a summary justifying RoB judgment, to ensure transparency in how these judgments were reached. We reassessed RoB based on the supporting comments reported in Cochrane RoB tables, in line with instructions from the Cochrane Handbook. Then, we compared our new assessments to judgments made by Cochrane authors. We compared the frequency of adequate judgments in reviews with two separate domains for blinding versus those with a joint domain for blinding. Results The total number of assessments for performance bias was 6918, with 8656 for detection bias and 3169 for the joint domain. The frequency of adequate assessments was 74% for performance bias, 78% for detection bias, and 59% for the joint domain. The lowest frequency of adequate assessments was found when Cochrane authors judged low risk - 47% in performance bias, 62% in detection bias, and 31% in the joint domain. The joint domain and detection bias domain had a similar proportion of specified outcome types (17% and 18%, respectively). Conclusions Splitting joint RoB assessment about blinding into two domains was justified because the frequency of adequate judgments was higher in separate domains. Specification of outcome types in RoB domains should be further scrutinized.