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Re: st: expand and duration model


From   ncdcta00@uniroma2.it
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: expand and duration model
Date   Mon, 18 Feb 2008 08:46:36 +0100

Thanks a lot Nick, it work perfect.


Catia
Quoting n j cox <n.j.cox@durham.ac.uk>:

This is not the problem tackled by -expand-. Nor does -expandby- (a
user-written program from SSC) claim to tackle anything like this
problem.

The problem has presumably been solved before, but off-hand I can't
remember any program that does it. However, a few lines should get
you most of the way.

expand 2
sort id datealt
by id: replace datealt = datebax[_n - 1] + 1 if mod(_n, 2) == 0
by id: replace datebax = datealt[_n + 1] - 1 if mod(_n, 2) == 0
by id: replace y = 0 if mod(_n, 2) == 0
by id: replace x = 0 if mod(_n, 2) == 0
by id: drop if _n == _N

Consider -id- 1.

1    11/02/2001       14/08/2003      100       205
1    02/03/2005        05/06/2005     120       130
1    13/08/2005        18/12/2005      100      130

We -expand 2- and then -sort- to get this:
                                                       _n
1    11/02/2001       14/08/2003      100       205     1
1    11/02/2001       14/08/2003      100       205     2
1    02/03/2005        05/06/2005     120       130     3
1    02/03/2005        05/06/2005     120       130     4
1    13/08/2005        18/12/2005      100      130     5
1    13/08/2005        18/12/2005      100      130     6

We need to work on the even-numbered observations, those with _n
of 2, 4, ... -- which are identified by -mod(_n, 2) == 0-.
We look backwards for the start date of each gap, and add 1; and
forwards for the end of each gap, and subtract 1.

A correction is that the last observation for each -id- will contain no
useful information, so we -drop- it. On the other hand, you might know
that your study ended at some time. Suppose your study ended on
31/12/2007. Then instead of

by id: drop if _n == _N

you should have

by id : replace datebax = mdy(12,31,2007) if _n == _N

You need to modify this if the recipe has to cope with gaps of length
0. Look for observations with -datebax < datealt- and -drop- them.

I have not tested this code, but I hope it helps.

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

Catia
=====

I have a problem to understand  how to expand my observation to stset
my data to use in a
duration model.
I have the following data set:

id  datealt          datebax          y          x
1    11/02/2001       14/08/2003      100       205
1    02/03/2005        05/06/2005     120       130
1    13/08/2005        18/12/2005      100        130
2..........
2..........

so I want  a dataset for the period that I don't observe the id:

id  datealt          datebax          y          x
1    11/02/2001       14/08/2003      100       205
1    15/08/2003        01/032005         0       0
1    02/03/2005        05/06/2005     120       130
1    06/06/2005        12/082005        0           0
1    13/08/2005        18/12/2005      100        130
2.........
2.......
2..........
Someone can help me?
I try with expandby but without results.

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Catia Nicodemo



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