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

    April 2025 in “ Journal of Clinical Epidemiology ”
    Josefina Salazar, Helene Moustgaard, Javier Bracchiglione, Asbjørn Hróbjartsson
    TLDR Non-blinded assessors tend to overestimate effects in trials by about 29%.
    This study investigated observer bias in randomized clinical trials by comparing outcomes assessed by blinded and non-blinded assessors. It found that non-blinded assessors exaggerated the effect of experimental interventions by about 29% on average compared to blinded assessors. The findings highlight the importance of blinding assessors in trials to minimize bias and ensure more reliable results.
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