Dear list
I have difficulties translating a SAS program into Stata. I am fairly
new to Stata, and also to survival analysis, but hope that I can get
some help from the list.
The situation from survival analysis. Events are registred in the
binary variable hd, and time-to-event in the continuous variable
year. The variables age1, age2, gender, c1 and c2 are binary
variables. The variable case is also binary.
This is the SAS program:
proc phreg data = test2;
model year*hd(0) = i1 i2 i3 age1 age2 gender c1 c2/risklimits ;
if year<1 and case=0 then i1=1;else i1=0;
if 1<=year<=2 and case=0 then i2=1;else i2=0;
if year>2 and case=0 then i3=1;else i3=0;
ods output parameterestimates = oddsm;
run;
As I interpret this, the baseline hazard rates are different in the
three intervals defined by i1 i2 and i3, adjusted for the remaining
covariates.
These are my Stata attempts:
/* Common to the code below */
gen i1=0
replace i1=1 if (year<1&case==0)
gen i2=0
replace i2=1 if (year>=1&year<=2&case==0)
gen i3=0
replace i3=1 if (year>2&case==0)
/* Attempt 1 */
stset year, failure(hd==1) /* note that SAS has the opposite hd==0 */
stcox i1 i2 i3 age1 age2 gender c1 c2
This does not work, since the SAS code implies different baseline
hazards in the three periods defined by inter1-3.
/* Attempt 2 */
stset year, failure(hd==1)
stcox i1 i2 i3 age1 age2 gender c1 c2, tvc(i1 i2 i3)
or
stcox age1 age2 gender c1 c2, tvc(i1 i2 i3)
I am not certain whether my use of tvc() is correct (and equivalent to
the SAS approach). At least, the estimates are very different.
/* Attempt 3 */
gen id=_n
stset year, failure(hd==1) id(id)
stcox i1 i2 i3 age1 age2 gender c1 c2
or
stcox i1 i2 i3 age1 age2 gender c1 c2, tvc(i1 i2 i3)
None of the attempts produce estimates anywhere near the output from
SAS. I am probably missing some options or have misunderstood their
use. I would appreciate help to get on.
I am using the latest Stata 9, updated to the most recent version.
Best,
Claus
_______________________________
Claus Dethlefsen, Msc, PhD
Statistiker ved Kardiovaskul�rt Forskningscenter
Klinisk Epidemiologisk Afdeling
Aalborg Sygehus Nord
Stengade 10, 2. sal
9000 Aalborg
Tlf: 9932 1086
9932 1094 (direkte)
Fax: 9932 1088
email: [email protected]
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