# Re: st: generate with varying number of previous observations

 From Maarten buis <[email protected]> To [email protected] Subject Re: st: generate with varying number of previous observations Date Sat, 26 Jan 2008 16:17:06 +0000 (GMT)

```--- William Bishop <[email protected]> wrote:

> I want to generate a variable by making calculations comparing the
> current observation to ALL the previous observations with the same
> id.
>
> Let's say I want to sum the difference in variable x for the
> observation 5 vs all the previous obs. It would look something like:
>
> gen diffx=(x[5]-x[4])+(x[5]-x[3])+(x[5]-x[2])+(x[5]-x[1])

Here is a solution without resorting to a loop:

*----------------- begin example -------------------
drop _all
input id obs x
1 1 2
1 2 5
1 3 6
1 4 8
1 5 11
2 1 3
2 2 4
2 3 6
2 4 7
2 5 9
2 6 11
end

sort id obs
by id (obs): gen diffx = _n*x - sum(x)
list
*----------------------- end example ------------------
(For more on how to use examples I sent to the Statalist, see
http://home.fsw.vu.nl/m.buis/stata/exampleFAQ.html )

Hope this helps,
Maarten

-----------------------------------------
Maarten L. Buis
Department of Social Research Methodology
Vrije Universiteit Amsterdam
Boelelaan 1081
1081 HV Amsterdam
The Netherlands

Buitenveldertselaan 3 (Metropolitan), room Z434

+31 20 5986715

http://home.fsw.vu.nl/m.buis/
-----------------------------------------

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```