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st: Survival analysis


From   "Westby, Christian Michael. (JSC-SK)[USRA]" <christian.westby@nasa.gov>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: Survival analysis
Date   Fri, 29 Oct 2010 09:04:11 -0500

Dear Statalisters, 

I am working on comparing survival times in one group of subjects before and after treatment and am having a hard time with the "stset" code. 


Using the following data set where testing was separated by 1 week, t is time of task before and after treatment (seconds) and ttrxt is time calculated to prevent time from being treated as continuous and fail is 0=completed, 1=not completed.



subjectid	treatment	fail			t	ttrxt
-----------------------------------------------------------------
1		pre		failed		169	169
1		post		failed		141	310
2		pre		failed		114	114
2		post		failed		84	198
3		pre		failed		564	564
3		post		failed		296	860
4		pre		failed		168	168
4		post		failed		332	500
5		pre		failed		215	215
5		post		failed		50	265
6		pre		completed		900	900
6		post		failed		196	1096
7		pre		completed		900	900
7		post		failed		298	1198
8		pre		completed		900	900
8		post		failed		280	1180
-----------------------------------------------------------------


I used 


. stset ttrxt, id(subjectid) failure(fail) exit(time .)


id:  subjectid
failure event:  fail != 0 & fail < .
obs. time interval:  (ttrxt[_n-1], ttrxt]
 exit on or before:  time .

------------------------------------------------------------------------------
       16  total obs.
        0  exclusions
------------------------------------------------------------------------------
       16  obs. remaining, representing
        8  subjects
       13  failures in multiple failure-per-subject data
     5607  total analysis time at risk, at risk from t =         0
                             earliest observed entry t =         0
                                  last observed exit t =      1198


I then ran


. stcox treatment, cluster(subjectid)

         failure _d:  fail
   analysis time _t:  ttrxt
  exit on or before:  time .
                 id:  subjectid

Iteration 0:   log pseudolikelihood = -20.175132
Iteration 1:   log pseudolikelihood = -18.079165
Iteration 2:   log pseudolikelihood = -18.026011
Iteration 3:   log pseudolikelihood = -18.025935
Refining estimates:
Iteration 0:   log pseudolikelihood = -18.025935

Cox regression -- no ties

No. of subjects      =            8                Number of obs   =        16
No. of failures      =           13
Time at risk         =         5607
                                                   Wald chi2(1)    =      4.22
Log pseudolikelihood =   -18.025935                Prob > chi2     =    0.0399

                              (Std. Err. adjusted for 8 clusters in subjectid)
------------------------------------------------------------------------------
             |               Robust
          _t | Haz. Ratio   Std. Err.      z    P>|z|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
   treatment |   4.610013   3.428317     2.05   0.040     1.073226    19.80218
------------------------------------------------------------------------------


I believe that the output and results are accurate however, I am unable to get Stata to correctly graph the survival curves using the following code



. stcurv, surv at1(treatment=0) at2(treatment=1)


the resulting graph incorrectly plots both groups starting at less than 100% at a time=0 and the x-axis scale is incorrect.


Any thoughts?



Chris

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