OVERDISPERSION IN CLINICAL MASTITIS DATA FROM DAIRY HERDS - A NEGATIVE BINOMIAL APPROACH

Cases of clinical mastitis on dairy farms were modelled using a negative binomial distribution. The negative binomial distribution is a gamma mixture of Poisson distributions, hence it is possible that the number of cases per farm had a Poisson distribution and the mean of these Poisson distributions had a gamma distribution. Risk factors associated with the number of cases per farm can be modelled with a negative binomial distribution but calculations involving the negative binomial likelihood are not straightforward. However, a very close approximation to the negative binomial is the Poisson model with an extra parameter for overdispersion. Fitting the Poisson model with overdispersion is illustrated.