We are applying a GzLM (using SPSS) for analysing the effects of two fixed categorical predictors (habitat type (2 levels) and seasons (4 leves), and their interaction, on data regarding counts per m2 (dv). Some samples have zeros, and we have detected overdispersion using a GzLM with Poisson error structure. We then proceed to use a negative binomial error distribution with log link, with a better fit of the model. My questions are:
(1) It is correct to run a GzLM for this kind of data where both predictors are categorical? We used to analyse this kind of data using GLM ANOVA with transformations (e.g. sqrt) but data is still non-normal, positive skewed.
(2) SPSS report the estimated marginal means and SE, it is ok to report these values in order to interpret the interaction?