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st: Time-varying covariates

From   Kyleigh Schraeder <>
Subject   st: Time-varying covariates
Date   Thu, 10 May 2012 14:30:05 -0400


I am running a cox regression model in survival analysis and I have a
question about time-varying covariates.  I have a covariate that
describes subjects total waiting time (in days) by the end of their
observation period.  At the beginning of the study (t=0), all subjects
were put on a wait-list.  I have two groups of subjects: 1) still
waiting by the end of the study and 2) waited and received help (i.e.,
stopped waiting during study).  I am interested in whether subjects'
waiting time impacts probability of the event of interest occuring.

I am wondering whether I should treat this waiting time covariate as
time-varying or not.  If I treat this as a time-varying covariate, I'm
looking for the appropriate syntax.  I tried the following syntax from
the example in the Stata manual under 'Cox regression with discrete
time-varying covariates' but I'm not sure if it's appropriate for my
Target event = Event_A
EventTime_A = Study time (time to event)
Waiting_Status = grouping variable

stcox   Wait_Time_A, tvc(Wait_Time_A) texp(.1*(_t))

Example of my datasetup in Stata:
id_1	Waiting_Status_A	Wait_Time_A	Event_A	EventTime_A
16	2	120	0	176
29	2	180	0	528
31	2	267	1	121
34	1	113	0	113
68	1	281	0	281

Thank you for your help!
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