This does not appear to be the same model as STATA. Your model (as documented) has dispersion (var/mean) of 1 + phi*mu, so you need to constrain phi = a/mu to get a constant dispersion. The output shown is not at all consistent with that.
On Thu, 30 Jun 2005, Lesnoff, Matthieu (ILRI) wrote: > Edward, you also can use the package aod on CRAN, see the help page of > the function negbin. > > Best > > Matthieu > > An example: > >> library(aod) >> data(dja) >> negbin(y ~ group + offset(log(trisk)), ~group, dja, fixpar = list(4, 0)) > > Negative-binomial model > ----------------------- > negbin(formula = y ~ group + offset(log(trisk)), random = ~group, > data = dja, fixpar = list(4, 0)) > Convergence was obtained after 113 iterations. > Fixed-effect coefficients: > Estimate Std. Error z value Pr(> |z|) > (Intercept) -0.5526 0.2277 -2.4267 0.0152 > groupTREAT -1.0205 0.2598 -3.9287 < 1e-4 > Overdispersion coefficients: > Estimate Std. Error z value Pr(> z) > phi.groupCTRL 0.8287 0.412 2.0117 0.0221 > Overdispersion coefficients set to fixed values: > Value > phi.groupTREAT 0 > Log-likelihood = -121.149; nbpar = 3; df.residual = 72; Deviance = 111.826; > AIC = 248.297 -- Brian D. Ripley, [EMAIL PROTECTED] Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595 ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
