test for significance of interaction in linear mixed models in nlme in R

Question

I use lme function in the nlme R package to test if levels of factor items has significant interaction with levels of factor conditi

Answer 1

The usual way to test if the interaction is significant is to do a likelihood ratio test (e.g. see discussion on R-Sig-ME).

To do that you have to also estimate a model without interaction and you'll also have to use method="ML":

f0 = lme(response ~ 0 + factor(condition) * factor(items),
    random = ~1|subject, method="ML")
f1 = lme(response ~ 0 + factor(condition) + factor(items),
    random = ~1|subject, method="ML")

You can then compare using anova:

anova(f0, f1)

Also see this blog post