Re: different result in two nb reg and pisson reg

Posted by Andy W on Sep 15, 2014; 12:14pm
URL: http://spssx-discussion.165.s1.nabble.com/different-result-in-two-nb-reg-and-pisson-reg-tp5727252p5727254.html

The coefficient estimates are not guaranteed to be exactly the same - but in practice they are typically quite close. (The mean function is the same for both equations, but the different variance functions basically weight observations differently.) I suspect the smaller the sample sizes the larger differences might be expected - especially if you have high leverage values in small samples.

The standard errors for negative binomial models will always be larger than for the equivalent Poisson model. So going from statistically significant to not statistically significant is always a possibility. If you post the model coefficients and their standard errors it would be easier to give armchair advice.