Hello: Im hoping the list can shed light on two items. I am using an nlme model to fit a logistic function and have these questions: 1) The model works fine using varIdent(~1|factor) and also using varComb(varIdent(~1|factor),varFixed(~covariate)), but not when varFixed(~covariate) is used alone. Using VarFixed alone gives the message: Error in recalc.varFunc(object[[i]], conLin) : dim<- : dims [product 153] do not match the length of object [846] In addition: Warning message: longer object length is not a multiple of shorter object length in: conLin$Xy * varWeights(object). Can anyone shed light on what this message means and how I might go about fixing the problem? I am still new to all this, so please explain with that in mind. 2) In my data, the mean of the variance of the within-group errors is dependent on a covariate, and in addition the variance of the variance is also dependent on a covariate. To model this would one use something like: varIdent(~varIdent(~1|covariate)|covariate)? I get the same error message as above when I try this or if I try: varIdent(~varFixed(~covariate)|factor). Is there a better way to do such nesting? Is it even possible? And again, what does the error mean? Thanks much in advance. John