Date:         Fri, 23 Sep 2005 16:11:37 +0930
Reply-To:     kylie.lange@flinders.edu.au
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         Kylie Lange <kylie.lange@flinders.edu.au>
Subject:      standardising weights in Poisson models?
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Hi all,

I have had a colleague explain the following problem to me. I hope it's
clear and that someone can offer some suggestions. If anyone has any
questions then I'll do my best to find out more info and clarify.

They are looking at incidence rates of certain kinds of injury, broken
down by age categories. They obviously have data on the number of
injuries (#inj) and the population size (#popn) for each age group, for
each of 9 years. In addition they have 'age standardised population
weights' (wght). These weights account for the fact that the age
distribution in the population changes over time (and I believe are
based on data from the last national census).

The usual way they use this information is to calculate #inj / #popn *
wght for each age-by-year combination. These weighted rates are then all
summed together to give an overall incidence rate.

They now would like to use this data in a Poisson regression model so
they can look at the rates over time and investigate possible trends
that may be present. The question is how to incorporate the
age-standardised weights into the Poisson model?

I'm afraid I don't know enough about how the weights are calculated to
know what is appropriate so I'm hoping that this will sound familiar to
someone who deals with this kind of data. I also may be using confusing
terminology but am happy to try clarify anything if needed.

Thanks for any suggestions or references to articles where this kind of
analysis has been carried out.

Kylie.