Hi, I have a problem with fitting a nonlinear mixed-effects model to my data. I was able to fit quite simple exponential model but now I?m trying to fit the following Gompertz model:> gomp<-nlme(paino~b0+b1*exp(-exp(-b2*(daydeg.scale-b3))), data=group1,fixed=b0+b1+b2+b3~1, start=c(b0=150, b1=3000, b2=0.6, b3=2500)) I?m getting the following error: Error in chol((value + t(value))/2) : the leading minor of order 2 is not positive definite Could you give me some ideas what might be wrong and how to deal with it? I know that the data and the syntax should be ok as I have tested it already with exponential model. The problem seems to be in the model-form. Many thanks, Niina Kotamaki University of Jyvaskyla
<nioksane at cc.jyu.fi> writes:> I have a problem with fitting a nonlinear mixed-effects model to my data. > I was able to fit quite simple exponential model but now I'm trying to fit > the following Gompertz model: > > > gomp<-nlme(paino~b0+b1*exp(-exp(-b2*(daydeg.scale-b3))), data=group1, > fixed=b0+b1+b2+b3~1, start=c(b0=150, b1=3000, b2=0.6, b3=2500)) > > I'm getting the following error: > > Error in chol((value + t(value))/2) : the leading minor of order 2 is not > positive definite > > Could you give me some ideas what might be wrong and how to deal with it? > I know that the data and the syntax should be ok as I have tested it > already with exponential model. The problem seems to be in the model-form.Try using the optional arguments verbose = TRUE, control = list(msVerbose = TRUE) in your call to nlme so that you can see how the parameter estimates are being changed. By the way, there is a self-starting model function SSgompertz that you could try instead. I would try to fit an nlsList model first with that.