Dear list members,

I wasn't sure whether to post this in the sig-mixed-models list, as it concerns the nlme package.

Suppose the following model:
model.lme=lme(distance ~ Sex*age, data=Orthodont,random=~age|Subject, weights=varIdent(form=~1|Sex),correlation=corExp(form=~age, nugget=T))

I would like to plot within-subject variability (e.g. the standard deviation of the fitted 'distance') over age. I expect that the vector of this variability (one per person per observed age?) is returned in model.lme, but I'm a bit confused with what to extract from the output.

Also regarding the nugget, should I initialise it to some value? Or just assume the value chosen by the model fitted are the actual 'measurement error' (please correct me if this is not what the nugget represents)?

Finally, am I right to think that heteroscedastic variances are still not possible with lme4?

I would appreciate any suggestions, I'm very new to mixed models.

Eleni Rapsomaniki
Epidemiology and Public Health
UCL, London, UK


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