1/4/2024 0 Comments Observer bias![]() Again, disagreements were resolved by discussion. Two investigators selected the outcomes independently. If several outcomes had been assessed under both blinded and nonblinded conditions, we preferred the primary outcome of the trial and the first assessment after the end of treatment (unless the primary outcome prescribed a different time point). We selected a single measurement scale from each trial. Disagreements were resolved by discussion. Two investigators read all other study reports and decided on eligibility. If a study was identified as potentially eligible for inclusion, we retrieved a full study report, which was read by an investigator who excluded all clearly ineligible studies. One investigator read all abstracts from standard databases and all text fragments from full-text databases. Our secondary objective was to examine reasons for variation in observer bias. Our primary objective was to evaluate the impact of nonblinded outcome assessment on estimated treatment effects in randomized clinical trials. We decided to systematically review trials with both blinded and nonblinded assessment of outcomes using the same measurement scales. They are frequently used as outcomes in clinical scenarios with no naturally distinct categories, and adjacent subcategories on a scale typically involve minor and vaguely defined differences. 8Īlthough subjective measurement scales such as illness severity scores are popular, they may be susceptible to observer bias. Our previous analysis of such trials with binary outcomes found substantial observer bias. 3, 7 Furthermore, the risk of confounding is considerable in indirect between-trial analyses, as “double-blind” trials may have better overall methods and larger sample sizes than trials that are not reported as “double-blind.”Ī more reliable approach involves analyses of trials that use both blinded and nonblinded outcome assessors, because such a within-trial design provides a direct comparison between blinded and nonblinded assessments of the same outcome in the same patients. 5, 6 However, such studies address blinding crudely because “double-blind” is an ambiguous term. Meta-epidemiological studies have compared double-blind trials with similar trials that were not double-blind. Unfortunately, the empirical evidence on observer bias in randomized clinical trials has been incomplete. However, the use of non-blinded assessors remains common, 2 – 4 especially in nonpharmacological trials for example, non-blinded outcome assessment was used in 90% of trials involving orthopedic traumatology 3 and 74% of trials involving strength training for muscles. 1īlinded outcome assessors are used in many trials to avoid such bias. Observer bias, sometimes called “detection bias” or “ascertainment bias,” occurs when outcome assessments are systematically influenced by the assessors’ conscious or unconscious predispositions - for example, because of hope or expectations, often favouring the experimental intervention. A failure to blind assessors of outcomes in randomized clinical trials may result in bias.
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