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st: troubleshooting code for simulating survival data

From   Michael McCulloch <>
To   Statalist <>
Subject   st: troubleshooting code for simulating survival data
Date   Wed, 28 May 2008 10:00:22 -0700

I've written the following do-file to simulate survival data, in the 
case of exponential distribution with one treatment variable (a) and 
no covariates. The code works when not in a program, and returns the 
specified Beta-coefficient (-0.8). May I ask for help in fine-tuning 
the program? I'm getting error message: "  'r(a)' evaluated to 
missing in full sample".

capture program drop sim_surv
	program define sim_surv, rclass
	tempname lambda a id t
	set obs 10 //create 10 observations
	gen `id'=_n //Create ID numbers
	set seed 1235 //set random number seed for reproducibility
	gen `a'=0 + int(2*uniform()) //Generate treatment: discrete 
0/1 uniform random variables
	gen `lambda'=-2
	gen `t'=(ln(uniform())/(`lambda'*exp(-0.8*`a'))) // using Bender, 2005
	stset `t'
	stcox `t'
		indeplist, local
		foreach var of varlist `X' {
			return scalar `var' = exp(_b[`var'])

set seed 12358
bootstrap a=r(a), eform reps(10): sim_surv

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