To the Editor: Dr Bassler and colleagues claimed that trials stopped for efficacy are prone to major bias. Unfortunately, their conclusions are based on faulty mathematical reasoning. We believe that they misinterpreted their data and provided incorrect guidance about ethical trial design.The authors' claim of a large bias is spurious because they have confused "bias" with the observation that results from trials addressing the same question will differ because of random variation. Consider an unbiased group of trials that go to completion. Trials with the largest effects will have similarly large interim effects, potentially leading to truncation. So comparing the truncated trials to the nontrun-cated trials is similar to comparing completed trials with large effects with those with lower effects. The difference the authors observed was both predictable and uninformative.
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