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st: stset help, please


From   Enrica Croda <croda@nicco.sscnet.ucla.edu>
To   statalist@hsphsun2.harvard.edu
Subject   st: stset help, please
Date   Wed, 27 Nov 2002 23:37:46 -0800 (PST)

Dear Stata-experts:

I am a newbie at survival analysis, and I would appreciate
your help with propoerly setting up the dataset for the analysis
with Stata 7.

I have annual data for 15 years from a panel household survey
on living arrangements of elderly households.

My original data come in the form of an _unbalanced_ dataset,
where the data are organized by ID and year (iis ID, tis year
in xt-language).

The data set covers the period 1984 through 1998 (15 years in
total, but elderly households are in my sample only if the head
of the household is older than 65. If they die or drop out of the
survey, I have no records for them after the death or drop-out.

Here is an example of how some records look like (ID 20302 is
particularly interesting, I think), where:

ID denotes elderly household id
year denotes year
indep denotes an indicator variable taking the value 1 if the elderly live
    independently in that year, and 0 otherwise.
age denotes age of the head of the household.


           ID       year      indep       age

          201         91          1        65
          201         92          1        66
          201         93          1        67
          201         94          1        68
          201         95          1        69
          201         96          1        70
          201         97          1        71
          201         98          1        72

         1101         84          1        78
         1101         85          1        79
         1101         86          1        80
         1101         87          1        81
         1101         88          1        82
         1101         89          1        83
         1101         90          1        84
         1101         91          1        85
         1101         92          1        86
         1101         93          1        87
         1101         94          1        88
         1101         95          1        89
         1101         96          1        90
         1101         97          1        91
         1101         98          1        92

         3401         84          1        73
         3401         85          1        74
         3401         86          1        75
         3401         87          1        76
         3401         88          1        77
         3401         90          0        79
         3401         91          0        80
         3401         92          0        81

        20302         87          1        65
        20302         88          1        66
        20302         89          1        67
        20302         90          0        68
        20302         91          0        69
        20302         94          0        72
        20302         95          0        73
        20302         96          0        74
        20302         97          1        75
        20302         98          1        76

        53501         84          1        84
        53501         85          1        85
        53501         86          1        86
        53501         87          1        87
        53501         88          1        88
        53501         89          1        89
        53501         90          0        90
        53501         91          0        91
        53501         92          0        92



For the moment, I am interested in studying the probability
that an elderly household moves out of independence at time
T, conditional on having been independent up to then.

So, trying to follow Stata terminology, I believe I have
multiple record data with delayed entry and right censoring.
I also may have gaps in between spells of independent living.

For now I think I would like to have "origin" and "entry" both
coincide with the case in which I first observe the elderly living
independently (indep==1), "failure" to be the case in which I
observe the elderly move out of independence (first time I observe
them with indep==0, after having observed indep==1).


My question is: how do I set up the st data set?


I understand that I first need to convert the snap-shot data
in time span data. I take care of this step by doing:

   " snapspan ID year `listvar', replace "

where listvar contains the list of variables I want to keep.

Next, I believe I need to do the stset step, but I cannot figure
out how to do it right.

The best I have come up with so far is:

   " stset year,
      id(ID) failure(indep==0) origin (indep==1) entry(indep==1) exit(time.)"

but I do not think it is correct. In case it matters, I get:


                id:  ID
     failure event:  indep == 0
obs. time interval:  (year[_n-1], year]
 enter on or after:  indep==1
 exit on or before:  time .
    t for analysis:  (time-origin)
            origin:  indep==1

------------------------------------------------------------------------------
    12617  total obs.
     1702  ignored because never entered
     1841  obs. end on or before enter()
------------------------------------------------------------------------------
     9074  obs. remaining, representing
     1285  subjects
      285  failures in multiple failure-per-subject data
     9237  total analysis time at risk, at risk from t =         0
                             earliest observed entry t =         0
                                  last observed exit t =        14

and then listing records for the above IDs:

           ID      indep       _st         _t0          _t        _d

          201          1         0           .           .         .
          201          1         1           0           1         0
          201          1         1           1           2         0
          201          1         1           2           3         0
          201          1         1           3           4         0
          201          1         1           4           5         0
          201          1         1           5           6         0
          201          1         1           6           7         0

         1101          1         0           .           .         .
         1101          1         1           0           1         0
         1101          1         1           1           2         0
         1101          1         1           2           3         0
         1101          1         1           3           4         0
         1101          1         1           4           5         0
         1101          1         1           5           6         0
         1101          1         1           6           7         0
         1101          1         1           7           8         0
         1101          1         1           8           9         0
         1101          1         1           9          10         0
         1101          1         1          10          11         0
         1101          1         1          11          12         0
         1101          1         1          12          13         0
         1101          1         1          13          14         0

         3401          1         0           .           .         .
         3401          1         1           0           1         0
         3401          1         1           1           2         0
         3401          1         1           2           3         0
         3401          1         1           3           4         0
         3401          0         1           4           6         1
         3401          0         1           6           7         1
         3401          0         1           7           8         1

        20302          1         0           .           .         .
        20302          1         1           0           1         0
        20302          1         1           1           2         0
        20302          0         1           2           3         1
        20302          0         1           3           4         1
        20302          0         1           4           7         1
        20302          0         1           7           8         1
        20302          0         1           8           9         1
        20302          1         1           9          10         0
        20302          1         1          10          11         0

        53501          1         0           .           .         .
        53501          1         1           0           1         0
        53501          1         1           1           2         0
        53501          1         1           2           3         0
        53501          1         1           3           4         0
        53501          1         1           4           5         0
        53501          0         1           5           6         1
        53501          0         1           6           7         1
        53501          0         1           7           8         1

--------------------------------------------------------------------------


Please let me know if you need fourther information.

Thank you very much in advance for any help you can give me.


Enrica Croda


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