# Negative Binomial dispersion parameter in SPSS

I am working in accident prediction modeling and I'm using SPSS Generalized Linear Model procedure with Negative Binomial distribution and Log Link Function.

Does anyone know the form in wich SPSS returns the dispersion parameter? Is it the form Var(x)=mu+dispersion*mu^2? All i know is that the dispersion parameter is equal to 0 for a Poisson distribution, where Var(x)=mu.

Yes you have the form right, specifically the variance for the $j$th observation is:
$$\text{Var}(x_j) = \mu_j(1 + au_j)$$
It can be tricky when reading texts. Sometimes people refer to the dispersion as the entire thing in the parentheses (which changes per the mean estimate of the variable), but other times they only refer to $a$ (which is what the software reports). I've written a blog post translating between a few of the regularly used notations for negative binomial parameterizations.