The association between mortality rate and putative
explanatory variables was investigated by negative binomial
regression, using the GENMOD procedure of the SAS software
(SAS Institute Inc.). Flocks slaughtered in the same
slaughterhouse were considered to be correlated, as they
often belonged to the same production organization, and
were transported by the same set of drivers. This clustering
was taken into account in a generalized estimating equation
(GEE) model in which the slaughterhouse was mentioned
in a repeated statement.