[R] main effect & interaction in 2-way ANOVA

[Peter Dalgaard]

array chip <arrayprofile at yahoo.com> writes:

> Hi,
> 
> I am just a little confused of mian effect in the
> analysis of variance (ANOVA) when you include or do
> not include an interaction term. Let's assume a simple
> case of 2-way ANOVA with 2 factors A and B, each with
> 2 levels. If it shows that main effect for A is
> significant when the interaction between A and B is
> NOT included, and the main effect for A is NOT
> significant when the interaction is included, what
> simply does this difference mean? I understand that
> main effect for A generally means averaging over
> levels of B, is this explanation for the situation
> when interaction is included or is not included or is
> irrelavant?
> 
> And if my interest is in the main effect of A, in the
> above senario, should I include the interaction (thus
> lose the significance) or not include the interaction
> (thus keep my significance)?
> 
> Thanks!

(Uh, oh, here we go again...)

As a number of people will probably soon point out, you shouldn't even
worry about that. 

Anyways, the results depend competely on parametrization. In the 2x2
layout with contr.treatment, you generally end up with "main effects"
of A testing differences within only one level of B.


-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
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