Re: st: RE: Estimation of differences of logistic regression models in three groups

 From roland andersson To statalist@hsphsun2.harvard.edu Subject Re: st: RE: Estimation of differences of logistic regression models in three groups Date Sat, 21 Feb 2009 11:07:32 +0100

```I have started by myselt but need feedback.

by the commands

lincom  _Ilapscopii_1+ _IlapXtim_1_2	,	hr
lincom  _Ilapscopii_1+ _IlapXtim_1_3	,	hr

I understand that this will give the estimates for the risk of
laparoscopy for timeperiod 2 and 3. The estimate for timeperiod 1 is
the baseline estimate from the model. The riskdifference is
significantly higher in the third timeperiod. Is this correct? How can
I make a test for the trend over time?

perpostkir~k  Odds Ratio   P>z	[95% Conf.	Interval]
_Ilapscopi~1	1.591794	0.104	.9093168	2.786497
(1)                    1.249688   0.373	.7650747	2.041264
(1)                    2.485883   0.000	1.759211	3.512719

Roland

> Thank you Kieran
>
> It should have been obvious for me to use interaction terms.
> I am not used to the lincom command. Can you guide how to use it with
> this result:
>
>
> komplik         Odds Ratio      P>z            [95% Conf.       Interval]
> _Ilapscopi              1.591794                0.104   .9093168        2.786497
> _Iperiod2               1.029331                0.858   .7497497        1.413169
> _Iperiod3               1.334           0.070   .9767403        1.821933
> _IlapXperiod2   .7850812                0.519   .3761646        1.638518
> _IlapXperiod3   1.561686                0.178   .816029 2.988697
>
> From this result I understand that laparoscopy has a higher rate of
> complications overall.
>
> 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.
>
> 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.
>>
>> Jönköping, sweden
>>
>> *
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>>
>>
>>
>> *
>> *   For searches and help try:
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>> *   http://www.ats.ucla.edu/stat/stata/
>>
>

*
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