- Homogeneity of variance/homoscedasticity is the assumption that the spread of outcome scores is roughly equal at different points on the predictor variable.
- This can be tested by looking at a plot of the standardized predicted values from your model against the standardized residuals (zpred vs. zresid).
- When comparing groups, this assumption can be tested with Levene’s test and the variance ratio (Hartley’s Fmax).
- If Levene’s test is significant (Sig. in the SPSS table is less than.05) then the variances are significantly different in different groups.
- Otherwise, homogeneity of variance can be assumed.
- The variance ratio is the largest group variance divided by the smallest. This value needs to be smaller than the critical values in the additional material.
Warning: There are good reasons not to use tests like Levene’s test. In large samples Levene’s test can be significant even when group variances are not very different. Therefore, it should be interpreted in conjunction with the variance ratio.