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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 02:17:13 +0100

I see!

-mark-'s been around for a long while, but not always. I'd guess
wildly it was introduced around version 4. Need to check on old
manuals.

It goes in [P] partly to reduce the pressure on [R], or so I guess.

Using -mark- interactively is perfectly good style. See also

SJ-7-2  dm0030  . . . . . . . . . .  Stata tip 44: Get a handle on your sample
        . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  B. Jann
        Q2/07   SJ 7(2):266--267                                 (no commands)
        tip on how to keep track of the observations that make
        up your estimation sample

On Wed, Jun 6, 2012 at 1:47 AM, Sarah Edgington <sedging@ucla.edu> wrote:

> Sorry that wasn't entirely clear.
> My intent was to have a flag for the observations that will be kept so that
> the data can be visually inspected to see that the right observations have
> been selected.  It doesn't actually drop the unwanted observations, though.
> For that you'd need to -keep if keep==1-.
>
> As an aside, has -mark- always been programming syntax?  I've long used it
> for creating dichotomous variables even when not defining a program.  I
> imagine I picked up the habit from someone's example code back when I was
> learning Stata (I think I started with version 7).  So I was a little
> surprised when I looked at the documentation recently and noticed that it's
> in the programming manual and not the base reference.  I guess the more
> standard way to make that sort of indicator would be - gen keep=(seq==max) -
> Using -mark- is going to be a hard habit to break, though.  (Note there's
> probably also a stylistic argument to be made for not using names like keep
> (or drop) when flagging observations to keep (or drop) to avoid confusion).

Nick Cox

> By
>
> mark keep if seq==max
>
> I imagine that Sarah meant
>
> keep if seq==max

 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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