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Re: st: snapsan command


From   sara bonfanti <sara.bonfanti@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: snapsan command
Date   Tue, 22 May 2012 10:54:58 +0200

Kieran and Steve,

thanks a lot for your explanations. I've understood the snapspan
command and solved my doubts.

sara

2012/5/22 Kieran McCaul <kieran.mccaul@uwa.edu.au>:
> ...
>
>
> Sara,
>
> I don't think you have understood -snapspan-.
>
> In the snapspan.dta example data, the variables x1 and x2 have been measured AT THE SPECIFIC TIMES recorded in the variable time.
>
> If I ignore this and simply -stset- the data prior to running a Cox model, the data will be set up incorrectly.
>
> To see this, look at the new variables _t0 and _t in the first listing below.  These are intervals of times - time-spans - within which the values of the variables x1 and x2 are fixed.  As you can see, for id=1 the data are saying that x1=10 and x2=20 in the time-span 0 to 5; x1=10 and x2=23 in the time-span 5 to 7; and x1=10 and x2=18 in the time-span 7 to 9.
>
> But this is wrong. x1=10 and x2=20 when time=5 and doesn't change until time=7 when x1=10 and x2=23 and doesn't change again until time=9 when x1=10 and x2=18.  Person id=1 has data recorded at three time points and these define two time-spans: 5 to 7 and 7 to 9.
>
> Consequently, when we set up the data for a Cox model using -stset- we want it to have two time-spans, not three. (See the second listing below).
> We don't know the value of x1 and x2 before time=5, so x1 and x2 will be missing for the time-span 0 to 5.
>
> The time-span 0 to 5 (for id=1) has been created by stset, because it is assuming that observation started at time=0, the default.
>
> Alternatively, I could instead use -stset time, f(event) enter(time) id(id)- and then for id=1, _t0 and _t will be missing prior to time=5. (the third listing below.)
>
>
> * 1. --- WITHOUT USING SNAPSPAN ---
> . webuse snapspan, clear
> . stset time, f(event) id(id)
> . list, sepby(id)
>
>     +---------------------------------------------------+
>     | id   time   x1   x2   event   _st   _d   _t   _t0 |
>     |---------------------------------------------------|
>  1. |  1      5   10   20       0     1    0    5     0 |
>  2. |  1      7   10   23       0     1    0    7     5 |
>  3. |  1      9   10   18       1     1    1    9     7 |
>     |---------------------------------------------------|
>  4. |  2      6    8   13       0     1    0    6     0 |
>  5. |  2     10    8   15       0     1    0   10     6 |
>  6. |  2     13    8   18       1     1    1   13    10 |
>     |---------------------------------------------------|
>  7. |  3      4    4   19       0     1    0    4     0 |
>  8. |  3      7    4   24       1     1    1    7     4 |
>     |---------------------------------------------------|
>  9. |  4      2    6   10       0     1    0    2     0 |
>  10. |  4      9    6   20       1     1    1    9     2 |
>     +---------------------------------------------------+
>
> * 2. --- USING SNAPSPAN ---
> . webuse snapspan, clear
> . snapspan id time event
> . stset time, f(event) id(id)
> . list, sepby(id)
>     +---------------------------------------------------+
>     | id   time   x1   x2   event   _st   _d   _t   _t0 |
>     |---------------------------------------------------|
>  1. |  1      5    .    .       0     1    0    5     0 |
>  2. |  1      7   10   20       0     1    0    7     5 |
>  3. |  1      9   10   23       1     1    1    9     7 |
>     |---------------------------------------------------|
>  4. |  2      6    .    .       0     1    0    6     0 |
>  5. |  2     10    8   13       0     1    0   10     6 |
>  6. |  2     13    8   15       1     1    1   13    10 |
>     |---------------------------------------------------|
>  7. |  3      4    .    .       0     1    0    4     0 |
>  8. |  3      7    4   19       1     1    1    7     4 |
>     |---------------------------------------------------|
>  9. |  4      2    .    .       0     1    0    2     0 |
>  10. |  4      9    6   10       1     1    1    9     2 |
>     +---------------------------------------------------+
>
> * 3. --- USING SNAPSPAN AND SPECIFYING ENTER() IN STSET ---
> . webuse snapspan, clear
> . snapspan id time event
> . stset time, f(event) enter(time) id(id)
> . list, sepby(id)
>
>     +---------------------------------------------------+
>     | id   time   x1   x2   event   _st   _d   _t   _t0 |
>     |---------------------------------------------------|
>  1. |  1      5    .    .       0     0    .    .     . |
>  2. |  1      7   10   20       0     1    0    7     5 |
>  3. |  1      9   10   23       1     1    1    9     7 |
>     |---------------------------------------------------|
>  4. |  2      6    .    .       0     0    .    .     . |
>  5. |  2     10    8   13       0     1    0   10     6 |
>  6. |  2     13    8   15       1     1    1   13    10 |
>     |---------------------------------------------------|
>  7. |  3      4    .    .       0     0    .    .     . |
>  8. |  3      7    4   19       1     1    1    7     4 |
>     |---------------------------------------------------|
>  9. |  4      2    .    .       0     0    .    .     . |
>  10. |  4      9    6   10       1     1    1    9     2 |
>     +---------------------------------------------------+
>
>
>
>
>
>
>
>
> ______________________________________________
> Kieran McCaul MPH PhD
> WA Centre for Health & Ageing (M577)
> University of Western Australia
> Level 6, Ainslie House
> 48 Murray St
> Perth 6000
> Phone: (08) 9224-2701
> Fax: (08) 9224 8009
> email: Kieran.McCaul@uwa.edu.au
> http://myprofile.cos.com/mccaul
> http://www.researcherid.com/rid/B-8751-2008
>
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