st: streg, enter and origin

[Henrik L A <henrikla@undermybed.dk>]

Dear Statalisters,

in writing my thesis, my (lack of) knowledge of Stata's -stset-  
function seem to have become a problem;  especially the options `enter'  
and `origin' cause confusion. (I have consulted the ST manual many  
times, but that did not help.)

My data is a stock sample of a population that is followed from  
randomisation on 1 January 1992 until 1 January 2005. I have data for  
date of birth (in the range form 1927 until 1969) and date of death  
(for those who die).

For the survival times, I have generated a variable called `survival'  
that counts the days of survival for an observation from day 0  
(1.1.1992) until day 4,749 (1.1.2005). For the censoring/failure issue,  
I have generated a dummy called `failure' that is equal to one for the  
observations who die, and zero otherwise. Finally, the month, day, and  
year of birth are stored in variables called `bm', `bd', and `by'.

The analysis I ultimately want to do is a Cox or a parametric  
regression with the likelihood function weighted by the survivor  
function to deal with the length-biased sampling issue. For this  
purpose I have -stset- my data like this:

. stset survival, failure(failure) origin(time mdy(bm,bd,by))  
enter(time mdy(1,1,1992))

     failure event:  failure != 0 & failure < .
obs. time interval:  (origin, survival]
 enter on or after:  time mdy(1,1,1992)
 exit on or before:  failure
    t for analysis:  (time-origin)
            origin:  time mdy(bm,bd,by)

------------------------------------------------------------------------ 
------
    92348  total obs.
        0  exclusions
------------------------------------------------------------------------ 
------
    92348  obs. remaining, representing
     4717  failures in single record/single failure data
 4.29e+08  total analysis time at risk, at risk from t =         0
                             earliest observed entry t =      8037
                                  last observed exit t =     28485

Above I have added a constant of 11,688 to the `survival'-variable,  
because the observations born after 1960 were excluded in a earlier  
version of my -stset- (11688 is = mdy(1,1,1992)); presumably because  
1.1.1960 means 0 to Stata and observations born after 1960 then ended  
up with negative survival times when I introduced `origin' (because t =  
`survival' - origin).

So, my question is if the procedure above is correct, and if not, if  
there is a better way to do the -stset-.



Kind regards,

Henrik Lindegaard
Aarhus, Denmark

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