I am having problems with defining the error terms for a two way repeated measures ANOVA in R. My data consists of wood density estimates for three radial positions (inner, middle, and outer) along a core extracted from a tree. There are a total of 20 tree species, 6 individuals of each species, and two cores from each tree.
To test the effect of radial position on wood density, I use the following two way ANOVA model with an error term that accounts for variability across individuals:
radpos.aov <- aov(WD ~ Species*Radialposition + Error(Individual), data=Radpos)
However, I am not sure if my specification of the error term is adequate. Am I also supposed to account for the variability within a core? To me this variability is the same as that due to radial position which is the main factor I am interested in.
Although I have devoted some time to reading about specifying the error term in Repeated measures ANOVA, I still have problems with practically specifying the error term. I will appreciate some help with this.