One of the usual assumptions for the GLM procedure is that the underlying errors are all uncorrelated with homogeneous variances. You can test this assumption in PROC GLM by using the HOVTEST option ...

This test has an assumption of normality for the different groups but also an assumption of homogeneity of variance or, in ...

The difference-from-nominal chart should be accompanied by a test that checks whether the variances for each product type are identical (homogeneous). Levene's test of homogeneity is particularly ...