The epidemiological unit was the flock slaughtered, that
is, a group of birds reared on the same floor and sent together
on the same day to the processing plant.
A two-stage sampling process was implemented monthly
to ensure random selection of the flocks to be included
for each slaughterhouse. First, a slaughter day and, second, a
sequence number within the schedule of the flocks to be
slaughtered on this day were randomly selected using the
SURVEYSELECT procedure of the SAS software (SAS Institute
Inc., USA). The number of flocks included monthly at each
slaughterhouse depended on the veterinary services available.
The overall number of flocks to be included was defined
according to statistical considerations described elsewhere