Interaction contrasts in repeated measures designs

Specific information concerning the nature of interaction effects in factorial designs may be obtained through the use of tetrad contrasts. Empirical familywise Type I error rates and power rates associated with 10 procedures for conducting tetrad contrasts in groups-by-trials repeated measures designs were obtained when the assumptions of multisample sphericity and multivariate normality were not satisfied. Only three procedures provided acceptable control of familywise Type I error rates under the assumption violation conditions. These procedures relied on a test statistic formed using an estimate of the standard error of the contrast which was based on only that data used in defining the contrast, in combination with either a Studentized maximum modulus, Hochberg (1988) step-up Bonferroni, or Shaffer (1986) modified sequentially rejective Bonferroni critical value. Minimal power differences between these three procedures were observed.