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This applet lets you explore the effect of violations of the assumptions of normality and homogeneity of variance on the type I error rate and power of t tests (and two-group analysis of variance).

After pressing the "begin" button, you will be shown two population distributions. By default, both populations are normally distributed with means of 0.0 and standard deviations of 1.0. The default sample size for simulated experiments is 5 subjects per group; the default alpha level is 0.05 and the default number of simulations is 2,000.

Simulated experiments can be conductedby pressing the "Simulate" button. Using the default values, 2,000 simulated experiments will be conducted for a situation in which the assumptions are met and the null hypothesis is true. Since the alpha level is 0.05, the proportion of experiments for which the null hypothesis is rejected should be about 0.05.

By changing the skew, you can investigate the effect of violating the normality assumption. By making the population standard deviations differ, you can investigate the effect of violating the assumption of homogeneity of variance. You can also modify the number of simulations and the type I error rate.

The simulations were developed as part of a grant from NSF to David Lane of Rice University. Partial support for this work was provided by the National Science Foundation's Division of Undergraduate Education through grant DUE 9751307. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.