Examination for Professional Practice of Psychology (EPPP) Practice Test

When is the violation of the homogeneity of variance assumption least likely to make statistical analysis invalid?

1. When groups contain the same number of participants

2. When the dependent variable is interval or ratio scaled

3. When alpha increases from .01 to .05

4. When a between-group design is used

The correct answer is: When groups contain the same number of participants

The homogeneity of variance assumption, which assumes that different groups in an analysis have similar variances, plays a crucial role in many statistical tests. When groups contain the same number of participants, any deviations in variance are less likely to distort the test results. This equality helps to stabilize the estimates of variance across groups, making it easier to detect any true differences between groups. Having equal sample sizes minimizes the impact of unequal variances (heteroscedasticity) on the overall outcome. That's why statistical tests are often more robust against violations of this assumption when the group sizes are the same. Other contexts, such as the level of measurement of the dependent variable or changes in alpha, do not inherently mitigate the potential for invalid results due to variance inequality. In contrast, using a between-group design might introduce additional complexities related to how variances are distributed across groups. Therefore, maintaining equal group sizes significantly aids in ensuring the validity of statistical analyses when the homogeneity of variance assumption is violated.