Hello, I am comparing 2 nonlinear models one with a correlation structure. I used the gnls command in the nlme package to fit model 1 and then the update(mod1,corr=corExp(form=~lat+lon,nugget=T) to fit the same model but with a correlation structure. I am comparing models using AIC, but I am getting results that don't seem to match. I get matching results for the model with no correlation structure, but non corresponding results for the model with a correlation structure. Below are logLik for the models along with additional information. Am I doing something wrong?????
Information for model 1 rss=sum(resid(mod1)^2)=1318.645 var=rss/n=1.654511 n=797 number of parameters (K) = 10 (this does include the variance estimate) if I use logLik(mod1) I get -1331.541 with 10 d.f. if I use the formula: (-.5*n*log(var))-(797/2*log(2*pi))-(n/2) from Burnham and Anderson 2002 page 12, I get the same result as above: -1331.541 Information for model 2 with a correlation structure corr=corExp(form=~lat+lon,nugget=T) rss=1338.92 n=797 var=rss/n=1.679950 K=12 logLik(mod2)=-1313.65 with 12 d.f. If I use the equation from Burnham and Anderson I get: -1337.621 different than the logLik result?? Dan ______________________________________________ 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.
