Fit an interaction between the surgery variable and the time period variable.
You can use -lincom- to estimate the surgery effect in each time period and use -testparm- to test significance of the interaction terms. This will test if the surgery effect is different across time periods.
Do the same thing for region.
______________________________________________
Kieran McCaul MPH PhD
WA Centre for Health & Ageing (M573)
University of Western Australia
Level 6, Ainslie House
48 Murray St
Perth 6000
Phone: (08) 9224-2140
Fax: (08) 9224 8009
email: [email protected]
http://myprofile.cos.com/mccaul
http://www.researcherid.com/rid/B-8751-2008
______________________________________________
The fact that no one understands you doesn't make you an artist.
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of roland andersson
Sent: Friday, 20 February 2009 9:47 PM
To: [email protected]
Subject: st: Estimation of differences of logistic regression models in three groups
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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