Re: different result in two nb reg and pisson reg

Posted by Andy W on Sep 16, 2014; 3:40pm
URL: http://spssx-discussion.165.s1.nabble.com/different-result-in-two-nb-reg-and-pisson-reg-tp5727252p5727273.html

As an exercise in curve fitting, I don't think you are likely to get a good fit from a count model. You can interactively superimpose different distributions on your histograms in SPSS and transform the X axis (e.g. log, power to .5 which is the square root). It is not out of the realm of possibility you could get a reasonable fit from a negative binomial model, but I'm very skeptical it would be possible with a Poisson model. The square root of the travel times seems sufficient to make the histograms pretty symmetric looking (and make predictions towards negative time values very unlikely within the sample), but I imagine you could make economic arguments for other transformations.

The residual plots don't look too bad -- but they are a bit more difficult to interpret for generalized linear models. I don't know what YouTube video you refer to. A good plot for any model is to superimpose the predicted distribution with the observed data, and I have an example of that here for negative binomial models, http://andrewpwheeler.wordpress.com/2014/02/17/negative-binomial-regression-and-predicted-probabilities-in-spss/, but that will be a bit annoying to extend to thousands of integer values.

Also note I said to transform time.travel to minutes - I imagine they were treated like seconds in the model already. This shouldn't affect the coefficient estimates, but (I think) it should make the intercept smaller by a factor of log(1/60), about [-4.1].