Dear all,
I am trying to fit a mixed-effects non linear regression, but I have some
trouble with it. My data are a balanced panel of 904 subjects with 8
observations (at regular periods) per subject.
The functional form of my model is Y=Aexp(-BX1)X2 +e. I want to allow
parameters A and B to vary among subjects and also include an autocorrelation
term. I have already fitted a standard nonlinear regression to the data, but I
keep having problems with NLME.
I have defined my data as a groupedData object, and when I try to fit the model
I get this error message:
dat<-groupedData(V~VAC|ID,data=dat)
attach(dat)
mod<-nlme(V~A*exp(B*YEAR)*VAC, fixed=A+B~1, random=A+B~1,
correlation=corCAR1(), start=c(A=1.2,B=0.2))
Error in getGroups.data.frame(dataMix, eval(parse(text = paste("~1",
deparse(groups[[2]]), :
Invalid formula for groups
Do you have any clue of what’s happening? It’s the first time I fit a model
like this in R, so the problem is probably pretty obvious, but I cannot see it.
Thank you very much,
Antonio
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