st: Producing incidence rates and weighted incidence rate

[KRISTIEN VERHEYEN <kristien.verheyen@aht.org.uk>]

Dear all, 

I have dataset from a cohort study including horses, trainers, number of
fractures and days at risk. Each observation consists of a horse with its ID
number, the trainer by which it was trained, the number of fractures it
sustained during the study period and the number of days it contributed to
the study.