There is an example at http://www.rpubs.com/johnhm/Overdispersed See Section 2.2 . John Maindonald email: john.maindonald at anu.edu.au<mailto:john.maindonald at anu.edu.au> On 22/05/2018, at 11:36, dani <orchidn at live.com<mailto:orchidn at live.com>> wrote: [[alternative HTML version deleted]]