A brief note on overdispersion
Poisson distribution assume variance is equal to the mean.
Quasi-poisson model assumes variance is a linear function of mean.
Negative binomial model assumes variance is a quadratic function of the mean.
#to test you need to fit a poisson GLM then apply function to this model
dispersiontest(object, trafo = NULL, alternative = c(“greater”, “two.sided”, “less”))
trafo = 1 is linear testing for quasipoisson or you can fit linear equation to trafo as well
c = 0 equidispersion
c > 0 is overdispersed
- Function description from vignette for AER package.
- Excellent StatsExchange description of interpretation.