🚀 go-pugleaf

>Mr. Keppel is quite wrong, or, more to the point, he is engaging in classic
>ANOVA interaction thinking.  An interaction variable is the product of two

Well, it seems to me that when you use an ANOVA, you engage in classic
ANOVA interaction thinking, otherwise you would use another technique.

The whole point to the ANOVA is the conditionality of the statement.  If
you have an interaction, you can't make an unconditional statement.  You
can always say something, and that is what simple main effects are for,
to allow you to make main-effect-like statements when the interaction
precludes making any unconditional statement.  Simple main effects are
statistically reasonable statements, usually constructed with an
appropriate power level.

If an interaction is present, there may be interesting overall main
effect differences and there may not be.  If the interaction is not
significant, and the main effect for A is, we can always be sure that at each
level of B, there is some interesting difference between the levels of
A.  This is the type of thinking usually done in ANOVA.

--
Paul Thompson, Ph.D.  |  Department of Psychiatry  |  (216) 844-7463
Case Western Reserve University  | Cleveland, OH  44106
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