st: From: [email protected]

 From [email protected] To [email protected] Subject st: From: [email protected] Date Thu, 31 Mar 2005 11:33:26 +0200

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 St�hl
PhD-student
Virology and Parasitology,
Dept. of Biomedical Sciences and Veterinary Public Health
Faculty of Veterinary Medicine and Animal Science
SLU
Uppsala, Sweden
[email protected]