# st: RE: RE: graph interaction in survival analysis

 From "Kieran McCaul" To Subject st: RE: RE: graph interaction in survival analysis Date Tue, 27 Oct 2009 06:21:32 +0800

```....

The at() options are used to produce curves at specific values of the
So if x and y are dichotomous and your model looks like this:
xi: stcox i.x*i.y

The model produces three beta coefficients, but Stata knows that there
are only two covariates and so four curves are produced using:
stcurve, survival at1(_Ix_1==0 _Iy_1==0)  ///
at1(_Ix_1==1 _Iy_1==0)  ///
at1(_Ix_1==0 _Iy_1==1)  ///
at1(_Ix_1==1 _Iy_1==1)

If both x and y are code 0 or 1 and you had instead generated the
interaction term by hand:
gen int_xy = x*y
and fitted the model:
xi: stcox i.x i.y i.int_xy

The model now contains three covariates because there is nothing to tell
Stata that -int_xy- is simply the interaction between x and y.
So then you would need to specify values for three two covariates to
produce the four curves:
stcurve, survival at1(_Ix_1==0 _Iy_1==0 _Iint_xy_1==0)  ///
at1(_Ix_1==1 _Iy_1==0 _Iint_xy_1==0)  ///
at1(_Ix_1==0 _Iy_1==1 _Iint_xy_1==0)  ///
at1(_Ix_1==1 _Iy_1==1 _Iint_xy_1==1)

______________________________________________
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) 9224-2701
Fax: (08) 9224 8009
email: Kieran.McCaul@uwa.edu.au
http://myprofile.cos.com/mccaul
http://www.researcherid.com/rid/B-8751-2008
______________________________________________
If you live to be one hundred, you've got it made.
Very few people die past that age - George Burns

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Weichle,
Thomas
Sent: Monday, 26 October 2009 11:46 PM
To: statalist@hsphsun2.harvard.edu
Subject: st: RE: graph interaction in survival analysis

When graphing the interaction using stcurve after fitting xi: stcox
i.x*i.y, isn't the interaction term also necessary in each of the at1()
options?

Tom Weichle
Math Statistician
Center for Management of Complex Chronic Care (CMC3)
Hines VA Hospital, Bldg 1, C202
708-202-8387 ext. 24261
Thomas.Weichle@va.gov

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```