OBJECTIVE: Knowledge of treatment assignment and failing to analyze results by randomized treatment groups -- an intention-to-treat analysis -- may cause bias in the treatment effect estimate in randomized controlled trials. This study was undertaken to determine the difference in lower limb muscle strength measured by blinded versus by unblinded outcome assessors in 73 progressive resistance strength training trials conducted in older adults. DESIGN: Retrospective analysis of randomized controlled trials published before 2007. RESULTS: Meta-regression analyses showed that trials that used blinded assessors (n = 18) tend to report smaller effect sizes than do those that used unblinded assessors (n = 55), with a difference of -0.80 (95% confidence interval -1.35 to -0.25). This result still holds even after adjusting for the use of an intention-to-treat analysis, with an adjusted difference of -0.65 (95% confidence interval -1.26 to -0.04). The reported effects were exaggerated in trials that used unblinded assessors. CONCLUSIONS: This study suggests that assessor blinding is important and is a safeguard to the internal validity of exercise trials in older adults.

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