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RE: st: Simple programming question?


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: Simple programming question?
Date   Wed, 18 Oct 2006 11:02:51 +0100

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

The bigger message remains the advice to -reshape-. 
This data structure will always be awkward for
most questions. 

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

Svend Juul
 
> Brad wrote:
>  
> I have data like the following:
> id day1 day2 day3 day4 day5 day6 ... day90
> 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
>  
> Assume that a 1 on day`i' represents failure.  I'm trying to 
> generate a
> variable that gives days to failure for survival analysis.  
> E.g., subject 1
> failed on day 4, subject 5 failed on day 5, subject 3 failed on day 1,
> subject 4 did not fail and would be considered censored.  90 
> days worth of
> data were collected but not all subjects were observed for 
> the full 90-day
> period.  Also note that subjects may be recorded as failing 
> (drinking at
> hazardous levels) on any combination of days during the 
> 90-day period.  I'm
> trying to calculate number of days to first failure....
> ---------------------------------------------------------------------
>  
> In the following example I use only 7 days, but the principle 
> is the same. To find the first day of changed status, examine 
> days in reverse order:
>  
> 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=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'>=.
> }
> list

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