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Re: st: RE: keep only one observation


From   Nick Cox <njcoxstata@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: RE: keep only one observation
Date   Wed, 6 Jun 2012 00:45:28 +0100

By

mark keep if seq==max

I imagine that Sarah meant

keep if seq==max

Nick

On Tue, Jun 5, 2012 at 10:45 PM, Sarah Edgington <sedging@ucla.edu> wrote:
> Lars,
> To get a sequence number within ID and date you'd want
> bysort id date_var : gen seq=_n
>
> The syntax you used makes sure that date_var is sorted within id but doesn't
> group by date when assigning seq.  If I understand you correctly, you
> actually want to define groups in terms of dates within IDs.
>
> To keep just the last observation for a date you could do
> bysort id date_var: keep if _n==_N
>
> That saves you the step of creating the seq number separately.  Of course,
> if you want to make sure you understand exactly what's happening you
> probably don't want to jump straight to that.  To see what's actually
> happening, try this:
> bysort id date_var : gen seq=_n
> bysort id date_var : gen max=_N
> mark keep if seq==max
>
> I think that gets you where you're trying to go.

Lars Folkestad

> I have a dataset of measured continuous data. The observations are from
> different participants. In my database i have 10.000+ observations.  I have
> an unique identifier pr participant. Some participants have been measured
> more than once in the observational period (on different dates). And for
> most participants i have more than one measurement pr date (for simplicity
> lets say they did not live up to strict quality measures and thus are
> rendered useless).
>
> I want to delete all other than the last observation pr participants on the
> different dates.
>
> I've tried to generate a date sequence variable (seq) by
>
> bysort id (date_var) : gen seq = _n
>
> But this does not give me what i want. I get a sequence number pr id but it
> does not seem to take into account the date_var.
>
> Fx
>
> What i get
> Id date seq
> 1 01.01.01 1
> 1 01.01.01 2
> 1 01.02.02 3
> 1 01.04.05 4
>
>
> What i want
> Id date seq
> 1 01.01.01 1
> 1 01.01.01 2
> 1 01.02.02 1
> 1 01.04.05 1
>
>
> So a two part question:
> How do i get stata to give a sequence variable that takes date and id into
> account?
> How do i then drop all but the last (the larges seq nr) from the list taking
> id into account?
>
> Hope this is somewhat clear.

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