Jay replied (to my comment): >Well it depends on the size of the effect doesn't it?

Yes. I should have been clearer. If the interactions are minute but statistically significant, then their effect on the main effects may be so small that odds ratios of A computed at each level of B could be essentially identical.

> In a large >dataset even a tiny interaction can be statistically significant, in >which case interpretation of main effects might still make good sense.

Yes. Unfortunately, with large enough data sets, one can get *everything* to be statistically significant. Even when the paramter estimates are so small that they have no scientific or business-case meaning.

>I've recently completed logistic regression analyses of tens of >thousands of medical records in which virtually every minuscule 3- & >4-way interaction was significant at the .001 level.

Ugh. You have my deepest sympathies. :-) :-)

David -- David L. Cassell mathematical statistician Design Pathways 3115 NW Norwood Pl. Corvallis OR 97330

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