# What is the best way to deal with over-dispersion in a poisson GLMM?

I am currently in the process of trying to complete a poisson GLMM analysis with two fixed (with an interaction) and two random effects using the glmer() function of the lme4 package. Using the testDispersion() function of the package DHARMa I found my data to be significantly over-dispersed (ratio = 1.877, p-value = <2.2e-16) so as a result attempted to use the glmer.nb() function in order to account for this over-dispersion by using the negative binomial distribution. My problem is that the model using this function still produced a significant dispersion test (ratio = 0.8817, p-value = 0.024). Should I still use this method to account for over-dispersion or is there a better way to account for it? The code for each of my models took the following forms:

Poisson: model1<-glmer(y~x1*x2 + (1|R1) + (1|R2), family = "poisson", data = dataset)

Negative binomial: model2<-glmer.nb(y~x1*x2 + (1|R1) + (1|R2), data = dataset)

• What is your research question ? Are you interested in inference or prediction ? What do the results of the two models tell you ? 0.88 is quite close to 1, so I'm not sure why you think that would be a problem ? Aug 9, 2020 at 12:54
• I fully agree with Robert (+1). One might want to consider observation-level random effects but that is a bit extreme given the non-severity of the observed over-dispersion. You might also want to use DHARMa::simulateResiduals and plot the residuals to get a better idea of how severe the problem is instead of focusing on ratio tests. A $p$-value of 0.024 is "non-issue" if the model adheres well to our modelling assumptions/research question. Aug 9, 2020 at 13:25
• +1 for suggesting the DHARMA method. I think it will be more relevant for the negative binomial model than the ratio test. Dec 20, 2021 at 1:56