sabwo <sabsiw <at> gmx.at> writes: [big snip; comparing aod::negbin and MASS::glm.nb fits]
> The thing i really dont understand is why there is such a big difference > between the deviances? (glm.nb = 30.67 and negbin=52.09?) Shouldnt they be > nearly the same?? > I don't have time to dig into this right now, but calculations of log-likelihoods or deviances often drop additive constants (such as the normalizing constant in a probability distribution), and different implementations often make different choices about which constant terms to include or not. If you dig around in the code you should be able to find out which terms are included or not (although admittedly this would be a nice thing to have included in the documentation). This does make it hard to compare across fits in different packages. The important thing (and the thing I'm fairly certain of, since I've used both packages and they both seem to be well-written) is that the **differences** in deviances when comparing models A and B both fitted in the same package should be the same (because the additive constants that are included or not cancel out in this case). 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.
