[R] Error in NLME (nonlinear mixed effects model)

[Lindsay Banin]

Hi there,

I have been trying to fit an NLME to my data. My dataset has two category levels - one is a fixed effect (level1) and one is a random effect (level2), though so far I have only experimented with the highest level grouping (fixed, level1), with the following code:

mod1 <- nlme(H ~ a*(1-exp(-b*D^c)),
data=sizes,
fixed=a+b+c~factor(Loc),
start=c(a=75,b=0.05,c=0.7))

This returns the error:
Error in getGroups.data.frame(dataMix, eval(parse(text = paste("~1", deparse(groups[[2]]),  : 
        Invalid formula for groups


Other points that it may be useful to note in diagnosing the problem are that:
1) I have tried both specifying Loc=as.factor and not specifying this.
2) I have tried other configurations of writing fixed=list(...) or fixed=c(...) and that always generates an error
3) I have not specified groupedData
4) My groups do not have equal sample sizes (unbalanced?)

Look forward to hearing back!
Thanks in advance,
Lindsay