st: From: karl.stahl@bvf.slu.se

[owner-statalist@hsphsun2.harvard.edu]

Hi all,
I have a problem that I can´resolve. I´m working on a survival model to describe bovine abortions using Cox proportional hazard. To model the effect of stage of gestation on the hazard of abortion I created a TVC using stsplit, splitting my data into three periods (trimesters) as follows:
*Setting up survival analysis
stset exit, id(ev_id) origin(time enter) failure(ev)
*Splitting data at days 90 and 180 to create variable trimester (trm)
stsplit ent_tr, at(90 180)
gen trm= 0
replace trm=1 if ent_tr>89
replace trm=2 if ent_tr>179
stset exit, id(ev_id) origin(time ent_tr) failure(ev)
xi: stcox nc_st   la_no   i.trm*nc_st

The stcox command gives me a point estimate of the hr or coef. of the trimester effect but does not give me estimates of std err or conf. int. How can this be? I did the same thing splitting the data into two periods and it worked fine.

Cox regression -- Breslow method for ties

No. of subjects =         1112                     Number of obs   =      3252
No. of failures =          138
Time at risk    =       242846
LR chi2(12)     =     38.60
Log likelihood  =   -923.21063                     Prob > chi2     =    0.0001

------------------------------------------------------------------------------
_t | Haz. Ratio   Std. Err.      z    P>|z|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
nc_st |   2.193517   1.152793     1.49   0.135     .7830613    6.144497
bvd |    .734624   .1280342    -1.77   0.077     .5220512    1.033754
_Ila_no_1 |   1.577242   .7896814     0.91   0.363     .5911874    4.207958
_Ila_no_2 |   3.579723   1.704545     2.68   0.007     1.407783    9.102553
_Ila_no_3 |   1.908593   .9152709     1.35   0.178     .7456198    4.885504
_Ila_Xnc_s~1 |   .3975807   .2300841    -1.59   0.111     .1278869    1.236017
_Ila_Xnc_s~2 |   .1534263   .0895957    -3.21   0.001     .0488458    .4819172
_Ila_Xnc_s~3 |   .3842764   .2126474    -1.73   0.084     .1299014    1.136773

_Itrm_1 | 1.603716 . . . . .
_Itrm_2 | 2.259726 . . . . .

_ItrmXnc_s~1 |   3.458197   1.431425     3.00   0.003      1.53645    7.783612
_ItrmXnc_s~2 |   2.718976   1.401987     1.94   0.052     .9896912    7.469835
------------------------------------------------------------------------------
end of do-file


If anyone could help me out I would be very grateful,

Karl Ståhl
PhD-student
Virology and Parasitology,
Dept. of Biomedical Sciences and Veterinary Public Health
Faculty of Veterinary Medicine and Animal Science
SLU
Uppsala, Sweden
Karl.Stahl@bvf.slu.se