Poisson regression using mid-year population estimates (census data)

Dear all

I am reasonably familiar with using Poisson regression as part of "stset" when I have dates individuals entering observation, dates they leave or the event happens and whether or not an event happens. Calculating rates, rate ratios etc and regressing based on another variables seems relatively straightforward.

However, the project I am doing is using cross-sectional data of hospital admissions over a 5 year period and seeing what number of individuals are admitted with the disease of interest (an "event"). Whilst calculating ratios using all admissions that year as a denoninator is ok, I have been advised to try and calculate a crude incidence rate based on mid-year population estimates from the hospital catchment area.

I have done this by dividing the number of "events" over the mid-year population estimate and obtaining a rate in 100,000 person-years. Is this correct? Additionally, how would I examine for trends over time using a rate with these data since "strate" commands won't work here.

Many thanks in advance.

Regards, D