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RE: st: Simple programming question?


From   "Anderson, Bradley J" <BAnderson1@lifespan.org>
To   "'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu>
Subject   RE: st: Simple programming question?
Date   Wed, 18 Oct 2006 09:16:34 -0400

I'd like to thank all who helped, especially Nick and Svend.  I had to adapt
a little to get things just right with my data but the example coding got me
over the hump and I think I've learned a couple of tricks to improve my
future Stata programming in the process.  Your gracious assistance is much
appreciated.

Brad

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu]On Behalf Of Nick Cox
Sent: Wednesday, October 18, 2006 6:34 AM
To: statalist@hsphsun2.harvard.edu
Subject: RE: st: Simple programming question?


If anyone is confused, Brad's question was 

"I'm trying to calculate number of days to first failure."

Failure I understand to be coded by 1. 
I was focusing on that question and that alone. 

Svend's code says 

If there is no failure (values are 0 or missing), 
the time of first failure is deemed to be the last day
with a non-missing value. And -event- is coded 0 
correspondingly. 

My code says, 

If there is no failure, the time of failure is returned
as missing. 

All I know about processing survival data can be written 
in one corner of a Kaplan-Meier plot, so I yield to Svend 
as evidently this is how survival people do it. 

Nick 
n.j.cox@durham.ac.uk 

Svend Juul
 
> Nick wrote:
> 
> Svend's technique is nice, but doesn't 
> produce the right answer if the event
> never occurs. In that case -time- is 
> initialised to 7, and never changed. 
> 
> A fix -- which by a interesting programming law 
> also yields shorter code -- is to initialise 
> -time- as missing. 
> 
>  clear
>  input id day1 day2 day3 day4 day5 day6 day7
>  1  0    0    0    1    0    0    0
>  2  0    0    0    0    1    1    .
>  3  1    1    1    1    1    1    1
>  4  0    0    0    0    0    0    0
>  5  0    0    .    .    .    .    .
>  end
>   
>  gen time= . 
>  gen event=0
>  forvalues I = 7(-1)1 {
>    replace time = `I' if day`I'==1
>    replace event = 1 if day`I'==1
>  }
>  list
> -------------------------------------------------
> 
> I don't fully agree: In the fifth observation my technique
> sets time to 2 and event to 0 (censoring), Nick's sets it to 
> missing despite the knowledge that the subject survived 
> at least two days. But when all time information is missing,
> the result should be missing, of course. This calls for a 
> slightly more, not less, complex series of commands:
> 
> gen time=7
> gen event=0
> forvalues I = 7(-1)1 {
>   replace time = `I' if day`I'==1
>   replace event = 1 if day`I'==1
>   replace time = `I'-1 if day`I'>=.
> }
> recode time(0=.)
> list
> 

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