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)
I don't think that attach'ing a groupedData object will propagate the grouping factors. I suggest that you avoid the construction of the groupedData object and also that you avoid attaching and detaching data frames. Just use
mod <- nlme(V~A*exp(B*YEAR)*VAC, dat, fixed=A+B~1, random=A+B~1|ID, correlation=corCAR1(), start=c(A=1.2,B=0.2))
You will also need to specify a formula for the corAR1 constructor. My guess is form = ~ YEAR | ID but you will need to decide if that is what you intend. Check the help page for corAR1 to see the list of possible arguments.
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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