Testing Homogeneity of Covariance Matrices

Figure 1. Box's test of equality of covariance matrices

Box's M is still significant, but the test statistic is much smaller.

Figure 2. Levene's test of equality of error variances

Levene's test for Log-cost has a significance value greater than 0.10, indicating that the equal variances assumption is not violated.

Figure 3. Spread vs. level plot for Treatment costs

The spread vs. level plot looks much better, and agrees with Levene's test. There is no apparent pattern in this plot, so there is no indication of a relationship here.