[R] Asking, are simple effects different from 0

From: Jarrett Byrnes <jebyrnes_at_ucdavis.edu>
Date: Tue, 04 Mar 2008 11:45:20 -0800


Hello, R-i-zens. I'm working on an data set with a factorial ANOVA that has a significant interaction. I'm interested in seeing whether the simple effects are different from 0, and I'm pondering how to do this. So, I have

my.anova<-lm(response ~ trtA*trtB)

The output for which gives me a number of coefficients and whether they are different from 0. However, I want the simple effects only, incorporating their intercepts, with their error estimates. Can I somehow manipulate this object to get that? Or, would a good shortcut be

my.simple.anova<-lm(response ~ trtA:trtB + 0)

and then use those coefficients and their error estimates?

If so, I realize that R gives me t tests for each coefficient. Now, for those I know I'm using the residual degrees of freedom. Would it then be more appropriate to use those, all with the same residual DF and apply a bonferroni correction, or, use the mean and SE estimate with the sample size for that particular treatment and perform an uncorrected one sample t-test to see if the value is different from 0?

Sorry for the bonehead question, but it's a situation I haven't seen before.

-Jarrett



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