Are StatXacts log-rank test and Wilcoxon-Gehan test results different from those obtained by the standard asymptotic method?
A. The exact logrank and exact Wilcoxon-Gehan tests in StatXact are linear rank tests based on permuting the generalized exponential and generalized Gehan scores, respectively, as specified in Kalbfleisch and Prentice (The Analysis of Failure Time Data, John Wiley, 1980, Section 6.2). With this choice of scores for the uncensored case, the logrank scores specialize to the usual Savage exponential scores test and the Wilcoxon-Gehan test specializes to the usual Wilcoxon rank-sum test. The logrank test in StatXact converts the censored survival data into logrank scores as described in Kalbfleisch and Prentice, then performs a two-sample linear rank permutation test on these scores. This is asymptotically equivalent to taking a 2×2 table at each failure time, but is not equivalent in small samples. Performance does differ between the exact and test and the standard asymptotic test. One popular implementation of the latter in the case of censored data, for example, uses the asymptotic vari
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