Using MASS::glm.nb (R) I am having problems of convergence with certain dataset, and it seems theta goes very high, resulting in NaNs "In sqrt(1/i)" during the fit. I am trying to understand this and have run some tests to see if NB is a significant better fit than Poisson, and it isn't (p~0.3). My question: is it still appropriate to use a NB when it isn't significantly better than a Poisson, keeping in mind that in this case is better to be conservative? Many thanks!
Also, if anyone knows how to fix those sqrt(1/i) errors that would be great. I can avoid them if I remove the covariates from the model. (I guess the covariates are explaining all of the dispersion?)