Six ways of modeling the transmission of two populations whose transmissibility is being compared.

<p>The dashed lines distinguish the models according the assumptions that are made about whether and the dispersion parameter are the same or different for the two populations. The axis on the right indicates the number of parameters used in each model. This is the sum of the number of parameters used to model (either 1 or 2) and the number of parameters used to model dispersion (either 0, 1 or 2).