Journal
STATISTICS IN MEDICINE
Volume 27, Issue 3, Pages 462-468Publisher
WILEY
DOI: 10.1002/sim.2912
Keywords
adjusting for ties; ordinal data; power calculation; sample size calculation; Wilcoxon-Mann-Whitney test
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In this paper we study sample size calculation methods for the asymptotic Wilcoxon-Mann-Whitney test for data with or without ties. The existing methods are applicable either to data with ties or to data without ties but not to both cases. While the existing methods developed for data without ties perform well, the methods developed for data with ties have limitations in that they are either applicable to proportional odds alternatives or have computational difficulties. We propose a new method which has a closed-form formula and therefore is very easy to calculate. In addition, the new method can be applied to both data with or without ties. Simulations have demonstrated that the new sample size formula performs very well as the corresponding actual powers are close to the nominal powers. Copyright (C) 2007 John Wiley & Sons, Ltd.
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