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I've read elsewhere that the VGAM package can be used to model underdispersed count data via the genpoisson families. However, when I look up the help file for genpoisson0, genpoisson1, and genpoisson2 they all say the following:

"In theory the λ parameter is allowed to be negative to handle underdispersion, however this is no longer supported, hence 0 < λ < 1."

"In theory the \varphi parameter might be allowed to be less than unity to handle underdispersion but this is not supported."

"In theory the α parameter might be allowed to be negative to handle underdispersion but this is not supported."

Where can I go to handle underdispersion?

Bryan
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1 Answers1

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  • You can use quasilikelihood methods, e.g. family=quasipoisson in glm() (in base R)
  • the glmmTMB package supports COM-Poisson (family = compois) and generalized Poisson (family = genpois) conditional distributions.

It's not clear to me whether the reasons discussed (briefly) here for why underdispersed generalized Poisson distributions are no longer supported in VGAM also apply to the implementation in glmmTMB ...

There is some discussion of the glmmTMB parameterizations/implementations of COM-Poisson and generalized Poisson in Brooks et al (2019).

Brooks, Mollie E., Kasper Kristensen, Maria Rosa Darrigo, Paulo Rubim, María Uriarte, Emilio Bruna, and Benjamin M. Bolker. “Statistical Modeling of Patterns in Annual Reproductive Rates.” Ecology 100, no. 7 (2019): e02706. https://doi.org/10.1002/ecy.2706.

Ben Bolker
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