Abstract
Approximate degrees of freedom omnibus and pairwise test statistics of Johansen (1980) and Keselman, Keselman, and Shaffer (1991), respectively, were used with numerous stepwise multiple comparison procedures (MCPs) to perform pairwise contrasts on repeated measures means. The MCPs were compared for their overall familywise rates of Type I error and for their sensitivity to detect true pairwise differences among means when multisample sphericity and multivariate normality assumptions were not satisfied. Results indicated that multiple range procedures which were modified according to the method described by Duncan (1957) were always robust with respect to Type I errors and were at least as powerful as the unmodified range procedures, and could result in increases in power as large as 22%. Overall, the Welsch (1977a) step-up, Peritz-Duncan (Peritz, 1970), and Ryan-Welsch-Duncan (Ryan, 1960; Welsch, 1977a) multiple range procedures were found to be most powerful.
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