Empirical Evidence of Observer Bias in Randomized Clinical Trials: Updated and Expanded Analysis of Trials With Both Blinded and Non-Blinded Outcome Assessors

This study analyzed 66 randomized clinical trials with 9,451 patients to investigate observer bias, finding that non-blinded outcome assessors exaggerated effect estimates by an average of 29% compared to blinded assessors. The pooled ratio of odds ratios (ROR) was 0.71, indicating more favorable estimates by non-blinded assessors, particularly in non-drug and industry-funded trials. The study emphasizes the importance of blinding outcome assessors to ensure reliable results, especially for subjective outcomes, and supports the methodological principle of blinding in clinical trials.