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. Copyright © 1990 John Wiley & Sons, Ltd.
