ezANOVA as my primary way of specifying mixed ANOVAs, I've hit a stumbling block when it come to adding a covariate to the model. I am using an ANCOVA in order to determine if there is a developmental trajectory in my data; namely, I need to be able to see the F-statistic and p-values for interactions with the covariate (see p.466 onwards here if you want an example).
ezANOVA, I can include covariates but the output does not show the F-statistic and p-values for interactions with the covariate - the main effect of the covariate is also not tested using this method.
ezANOVA model is as follows:
aov.model<-ezANOVA( data=textureView.child.outliersRemoved , dv=.(x) , wid=.(ID) , within=.(Texture,View) , between=.(TNOGroup) , between_covariates=.(Age) , type=3 , return_aov=TRUE )
Another option is to use
Anova, but I don't know how to specify the error terms properly for either and I'm limited because I want to use Type-III sums of squares (
drop1 doesn't work in the cases where I've tried to use the
aov wrapper for
lm; it fails while reporting
'Error in formula.default(object, env = baseenv()) : invalid formula').
Finally, I've heard about using the
nlme package to specify my ANCOVA as a mixed model instead, but I don't know where to begin here (despite spending a while reading about it).
To give a summary, I'm trying to do a 2 (between; TNOGroup) x 2 (within, Texture) x2 (within, View) mixed ANCOVA, with age as a covariate. I want to use Type-III sums of squares, and see the F-statistic and p-values for interactions with the covariate, as well as for the main effect of the covariate.
Any advice on the best way to do this would be much appreciated.