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Re: st: snapsan command
From
sara bonfanti <[email protected]>
To
[email protected]
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 <[email protected]>:
> ...
>
>
> 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: [email protected]
> http://myprofile.cos.com/mccaul
> http://www.researcherid.com/rid/B-8751-2008
>
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