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In a randomized placebo-controlled double-blind trial of 230 congestive heart failure patients, four treatments were evaluated for efficacy, with exercise tolerance time (ETT) as the primary outcome. Various two-sample tests were applied to the analysis of ETT data. It is shown in this paper that the conventional two-sample tests (t and rank-sum) are insensitive to situations where the effect of the experimental therapy is not consistent across a patient population. Tests recommended by O'Brien are more appropriate for these data. It is also shown that the application of the O'Brien tests led to the identification of sub-groups where the observed effect of the experimental therapy was most pronounced.

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