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Re: st: _N in by-groups


From   Phil Clayton <philclayton@internode.on.net>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: _N in by-groups
Date   Fri, 19 Aug 2011 18:02:56 +1000

. program dispby, byable(recall)
  1. marksample touse
  2. count if `touse'
  3. end

. bys foreign: dispby

---------------------------------------------------------------------------------------------------------------
-> foreign = Domestic
   52

---------------------------------------------------------------------------------------------------------------
-> foreign = Foreign
   22

This is documented in [P] byable

Phil

On 19/08/2011, at 5:41 PM, Matthew White wrote:

> So if I execute:
> sysuse auto
> bys foreign: drop if _n == _N
> Then two observations are dropped because _N is the number of
> observations in the by-group.
> 
> But in this (admittedly silly) example, _N seems to be the number of
> observations in the data set:
> program dispby, byable(recall)
> disp `0'
> end
> sysuse auto
> bys foreign: dispby _N
> Both times 74 is displayed, instead of 52 in the first by-group and 22
> in the second.
> 
> Thanks,
> Matt
> 
> On Fri, Aug 19, 2011 at 10:07 AM, Maarten Buis <maartenlbuis@gmail.com> wrote:
>> On Fri, Aug 19, 2011 at 2:21 AM, Matthew White wrote:
>>> If a Stata command has by-groups, it seems like _N is interpreted
>>> sometimes as the number of observations in the by-group and sometimes
>>> as the number of observations in the data set.
>> 
>> If you use the -by :- prefix it is always defined as the number of
>> observations within each by-group. Stata would be a pretty lousy
>> program if such a scalar randomly changed meaning...
>> 
>> If you want the total number of observations than I would just do:
>> 
>> local Ntot = _N
>> 
>> or inside a program that previously used -marksample-:
>> 
>> count if `touse'
>> local Ntot = r(N)
>> 
>> later you do
>> 
>> by <somevar> `touse' : <something using _N and `Ntot'> if `touse'
>> 
>> Hope this helps,
>> Maarten
>> 
>> --------------------------
>> Maarten L. Buis
>> Institut fuer Soziologie
>> Universitaet Tuebingen
>> Wilhelmstrasse 36
>> 72074 Tuebingen
>> Germany
>> 
>> 
>> http://www.maartenbuis.nl
>> --------------------------
>> *
>> *   For searches and help try:
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>> 
> 
> 
> 
> -- 
> Matthew White
> Evaluation Coordinator
> Urban Micro-Insurance Project
> Innovations for Poverty Action
> 
> +254 (0)701 025 276
> mwhite@poverty-action.org
> 
> *
> *   For searches and help try:
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> *   http://www.ats.ucla.edu/stat/stata/


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