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Re: st: Time-varying covariates

From   "Weichle, Thomas" <>
To   <>
Subject   Re: st: Time-varying covariates
Date   Thu, 10 May 2012 15:32:36 -0500

You may want to look into the command -stplit- for creating time-varying
covariates for discrete data.

Suppose Waiting_Status is a discrete variable with 0 = still waiting, 1
= received help.  The variable Wait_Time_A is the time from the
beginning of the study until a person received help.  You would split
the data at the point in which a person received help.  Prior to this
point, in the time spectrum, this person had a Waiting_Status value of
0.  After this point, this person would have Waiting_Status value of 1
and would continue to be categorized as 1 until event or censoring.

stsplit Waiting_Status_tvc, at(0) after(time = Wait_Time_A)
replace Waiting_Status_tvc = Waiting_Status_tvc + 1 if Waiting_Status ==

Only individuals that received help would be split resulting in multiple
records for this individual.  Waiting_Status_tvc is the new time-varying
covariate which is created.  You might consider examining the effect of
Waiting_Status_tvc on time to event.

stcox Waiting_Status_tvc

Tom Weichle
Math Statistician
VA Information Resource Center (VIReC)
Edward Hines Jr. VA Hospital, Bldg 18, 202E
708-202-8387 ext. 24261 

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