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There is also a trend towards an increasing frequency of complications
for both surgical methods. How can I test for the trend over the 3
time period?

There is also maybe a trend for increasing complication rate for
laparoscopy. I would like to test for trend also here.

It is anyhow important that the time trend is towards an increasing
risk and not decreasing, which should be expected.

I would like to estimate the risk of complication for laparoscopy
compared to open surgery for each timeperiod. Is laparoscopy
associated with a higher risk in all time periods?

I would appreciate your help.

Roland Andersson
surgeon, ass prof
Jönköping, Sweden


2009/2/21 Kieran McCaul <kamccaul@meddent.uwa.edu.au>:
> 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: kamccaul@meddent.uwa.edu.au
> 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: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of roland andersson
> Sent: Friday, 20 February 2009 9:47 PM
> To: statalist@hsphsun2.harvard.edu
> 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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