Chuanjun Zhang <zhangchu <at> umkc.edu> writes: > > Dear R Users: > > I am trying to compare several structures of the within-patient covariance > such as unstructured, Autoregressive, and spatial by using the MIXED effects > model. Can AIC, BIC be negative ? If yes, then in what situations they may > be negative. >
This almost deserves to be a FAQ, although it's a statistical rather than an R issue: http://finzi.psych.upenn.edu/R/Rhelp02a/archive/46734.html http://finzi.psych.upenn.edu/R/Rhelp02a/archive/108660.html The one-sentence answer is that since probability *densities* can be >1, log-likelihood *densities* can be >0 and hence negative log-likelihoods can be <0 (so AIC/BIC can also be <0). However, there's a potentially larger issue with using AICs for mixed models, which is that it's not always entirely clear what the right number of degrees of freedom is for a random effect ... cheers Ben Bolker ______________________________________________ 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.
