basic question about anova

Hi all,

I have a one-way ANOVA with N=16 for each of two factor levels. The
factor is just significant (p = around 0.04). There is another two-
level factor which I can add, making a balanced 2x2 ANOVA with N=8 in
each cell. This factor was not expected to be a significant predictor
and indeed it is not. However, adding in this second factor has the
effect of slightly increasing the p value for the first factor to
around 0.05.

I'm trying to understand how this can happen. I can think of two
reasons which kind of make sense to me but I'm not sure. Perhaps the
interaction term explains a little bit of the variance that the first
factor was previously explaining? It can't be the second factor,
because it is orthoganal to the first, right?

Or maybe, I wonder, does a two-way ANOVA inherently have a little less
power than a one-way?

Cheers,

Ben

.

Relevant Pages

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