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AIC and BIC in mixed effects model
2 messages · chuanjun zhang, Ben Bolker
1 day later
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