Dear Ben, Thanks for your useful information. Now I know at least what I do not want ;-)
The sas-code that corresponds to what I want is: proc nlmixed data=stab.BatchdataParameter ; parms Ea=92 A0=93 A1=34.5 e=10 s2=10 A11=1 A12=1; k= exp(A1)*exp(-Ea/(.0083144*tempK))*exp(eps1)*exp(A11*(Process='Process 1') + A12*(Process='Process 3')); m= A0*exp(-k*time); model result ~ normal(m,e); random eps1~normal(0,s2) subject=batch; run; So I defenitely want that A1 varies (fixed) across Processes (like in the sas-code above, there are three processes). The other attempts were just a selection of what I was trying to make things work and to understand better what the function nlme was doing. I checked the NAs in my data and no NAs are present so the problem might be something else. Could it be that I only have One batch for Process 2 and Process 3 while I'm having five batches for Process 1? Sas is not having problems with that... Thanks again for you quick reply. -- View this message in context: http://r.789695.n4.nabble.com/Add-covariate-in-nlme-tp4567189p4570177.html Sent from the R help mailing list archive at Nabble.com. ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.