Dale McLerran wrote: > In some experiments, you may run into small numbers of observations > for at least some combinations of factors. I think you could get > into trouble performing a test for homogeneity of variance when > some groups have few observations.

A good point.

> Would you have any qualms > with fitting the full model, generating the residuals, and then > performing an HOVTEST on the residuals for each factor separately? > This may not be an optimal test, but it seems to me that it > should suffice for most problems.

This is one reason why I like the Brown-Forsythe and Fligner-Killeen tests, as well as the *adjusted* Bartlett's test if those are not available. You get real robustness with these [as well as with Levene which has a different problem]. Note that SAS does not do the latter two. Your suggestion seems reasonable to me, although we would have to watch the alpha for the multiple [non-independent] tests.

We might be better off just doing residual plots...

David -- David Cassell, OAO Corp. Cassell.David@epa.gov Senior computing specialist mathematical statistician