st: troubleshooting code for simulating survival data

[Michael McCulloch <mm@pinest.org>]

Hello,
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".

clear
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'])
		}

end
set seed 12358
bootstrap a=r(a), eform reps(10): sim_surv
*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/