By creating interaction terms involving products of the dummies for time periods and surgical method, and likewise between dummies for time periods and regions, you can easily test for changes over time in regional differences and between surgical methods. David Greenberg, Sociology Department, New York University
----- Original Message -----
From: roland andersson <[email protected]>
Date: Friday, February 20, 2009 7:49 am
Subject: st: Estimation of differences of logistic regression models in three groups
To: [email protected]
> I am analysing the frequency of surgical complicationafter
> laparoscopic compared to open appendectomy in alrge national sample of
> patients.
>
> I use logistic regression to adjust for differences in comorbidity,
> age and sex between the two surgical methods. I obtain a higher risk
> of complications after laparoscopic appendectomy ( OR 1.97).
>
> I know want to analyse if this risk-difference between laparoscopic
> and open appendectomy has decreased between three timeperiod, and
> between three geographical regions.
>
> Shall I put the timeperiods and regions in the same model and use some
> postestimation method to do the inference? Or shall I do separate
> calculations for each timeperiod and geographic region? If so how can
> I estimate the difference in risk between the timepriods and make the
> statistical inference?
>
> Here is a simplified model with the important variables:
>
> xi:logistic perpostkirkomplik lapscopi i.appdgn i.agekat sex i.period
> i.region comrbidityvariables, or
>
> I assume this can be done with some postestimation methods but I need
> your helpt o understand them.
>
> Roland Andersson
> Jönköping, sweden
>
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