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Date:         Fri, 23 Sep 2005 10:12:47 -0300
Reply-To:     Hector Maletta <hmaletta@fibertel.com.ar>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         Hector Maletta <hmaletta@fibertel.com.ar>
Subject:      Re: standardising weights in Poisson models?
Comments: To: kylie.lange@flinders.edu.au
In-Reply-To:  <4333A3A1.9020105@flinders.edu.au>
Content-Type: text/plain; charset="iso-8859-1"

Kylie, Standardising the age distribution to conform to the national demographic data is a common practice. There may be some objections to this practice in general, but if the practice is accepted in general I do not see any reason why you shouldn't use weights specifically in your Poisson model. You simply incorporate a variable reflecting those weights and use it as your weighting variable. For this the general procedure is as follows:

1. Create a variable with the proportion of population in the respective age group of each case. Example: IF (AGE < 15)POP=0.23. IF (AGE GE 15 AND AGE LE 24)POP=0.14. .....................[CONTINUE FOR OTHER GROUPS; VALUES SUGGESTED ARE ONLY EXAMPLES, USE ACTUAL VALUES FROM CENSUS.]

2. Create a variable with the proportion of cases in each age group WITHIN YOUR SAMPLE: IF (AGE < 15)sample=0.32. IF (AGE GE 15 AND AGE LE 24)sample=0.18. .....................[CONTINUE FOR OTHER GROUPS; VALUES SUGGESTED ARE ONLY EXAMPLES, USE ACTUAL VALUES FROM YOUR SAMPLE].

3. COMPUTE THE RATIO OF THE ABOVE TWO VARIABLES:

COMPUTE AGEWGT=POP/SAMPLE.

4. Adopt this variable for weighting:

WEIGHT BY AGEWGT.

5. PROCEED WITH YOUR POISSON MODEL.

Hector

> -----Original Message----- > From: SPSSX(r) Discussion [mailto:SPSSX-L@LISTSERV.UGA.EDU] > On Behalf Of Kylie Lange > Sent: Friday, September 23, 2005 3:42 AM > To: SPSSX-L@LISTSERV.UGA.EDU > Subject: standardising weights in Poisson models? > > 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. > > __________ Información de NOD32 1.1230 (20050922) __________ > > Este mensaje ha sido analizado con NOD32 Antivirus System > http://www.nod32.com > >


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