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st: Interpretation of tvc in stcox


From   Jennifer Evans <evansjenniferl@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: Interpretation of tvc in stcox
Date   Wed, 7 May 2008 14:46:29 -0700 (PDT)

Dear statalist,
   
  I have a dataset with multiple records per person with time varying covariates. I am running
survival analysis with the tvc option in stcox and have found 2 variables with significant time
interactions.  My questions are:
(1) Am I setting up the stcox statement correctly (see code below)
(2) Regarding the interpretation- Is it correct to say that the variable var1 has a hazard of 0.35
with the relative hazard then decreasing by a factor of 1.00 with each unit of _t?   

Any comments or suggestions would be appreciated! 

Best regards,
Jennifer Evans


. xi: stcox yrsinj_b age_b gender_b var1 var2 var3 var4,  nolog noshow tvc(var1 var3) 

Cox regression -- Breslow method for ties

No. of subjects =          360                     Number of obs   =      1577
No. of failures =          104
Time at risk    =       226907
                                                   LR chi2(16)     =    117.25
Log likelihood  =   -467.29121                     Prob > chi2     =    0.0000

------------------------------------------------------------------------------
          _t | Haz. Ratio   Std. Err.      z    P>|z|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
rh           |
    yrsinj_b |   1.033031    .029971     1.12   0.263     .9759283    1.093476
       age_b |   .9916956   .0319112    -0.26   0.796     .9310822    1.056255
    gender_b |   1.114194   .2912036     0.41   0.679     .6675631    1.859641
        var1 |   .3489511   .1266303    -2.90   0.004     .1713457    .7106501
        var2 |   .3903011   .1237031    -2.97   0.003     .2097093    .7264101
        var3 |   .2174367   .0915801    -3.62   0.000     .0952409     .496412
        var4 |   1.562432   .3349837     2.08   0.037     1.026371    2.378471
-------------+----------------------------------------------------------------
t            |
        var1 |   1.001592    .000593     2.69   0.007      1.00043    1.002755
        var3 |   1.001354   .0005982     2.26   0.024     1.000182    1.002527
------------------------------------------------------------------------------





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