[R] Help please! How to code a mixed-model with 2 within-subject factors using lme or lmer?
mark_difford at yahoo.co.uk
Sun Sep 14 15:36:21 CEST 2008
>> but I can't figure out the /(Lobe*Tissue) part...
This type of nesting is easier to do using lmer(). To do it using lme() you
have to generate the crossed factor yourself. Do something like this:
tfac <- with(vslt, interaction(Lobe, Tissue, drop=T))
mod2<-lme(Volume ~ Val*Lobe*Tissue, random = ~1|Subject/tfac, data = vslt)
Pre-Scriptum: You can also use ?":" but ?interaction is more flexible and
roberto toro wrote:
> I'm using aov() to analyse changes in brain volume between males and
> females. For every subject (there are 331 in total) I have 8 volume
> measurements (4 different brain lobes and 2 different tissues
> (grey/white matter)). The data looks like this:
> Subject Sex Lobe Tissue Volume
> subect1 1 F g 262374
> subect1 1 F w 173758
> subect1 1 O g 67155
> subect1 1 O w 30067
> subect1 1 P g 117981
> subect1 1 P w 85441
> subect1 1 T g 185241
> subect1 1 T w 83183
> subect2 1 F g 255309
> subect2 1 F w 164335
> subect2 1 O g 71769
> subect2 1 O w 31879
> subect2 1 P g 120518
> subect2 1 P w 90334
> subect2 1 T g 168413
> subect2 1 T w 75790
> subect3 0 F g 243621
> subect3 0 F w 167025
> subect3 0 O g 65998
> subect3 0 O w 29758
> subect3 0 P g 118026
> subect3 0 P w 91903
> subect3 0 T g 156279
> subect3 0 T w 82349
> I'm trying to see if there is an interaction Sex*Lobe*Tissue. This is
> the command I use with aov():
> Subject is a random effect, Sex, Lobe and Tissue are fixed effects;
> Sex is an outer factor (between subjects), and Lobe and Tissue are
> inner factors (within-subjects); and there is indeed a significant
> 3-way interaction.
> I was told, however, that the results reported by aov() may depend on
> the order of the factors
> (type I anova), and that is better to use lme() or lmer() with type
> II, but I'm struggling to find the right syntaxis...
> To begin, how should I write the model using lme() or lmer()??
> I tried this with lme():
> but I have interaction terms for every level of Lobe and Tissue, and 8
> times the number of DF I should have... (around 331*8 instead of
> Using lmer(), the specification of Subject as a random effect is
> but I can't figure out the /(Lobe*Tissue) part...
> Thank you very much in advance!
> R-help at r-project.org mailing list
> PLEASE do read the posting guide
> and provide commented, minimal, self-contained, reproducible code.
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