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Re: st: _N in by-groups


From   Maarten Buis <maartenlbuis@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: _N in by-groups
Date   Fri, 19 Aug 2011 10:20:52 +0200

See -help byable-: With the -recall- the -by-ing- is done through
-marksample- and -if-, so within that program _N is, and should be,
the numer of observations in the entire dataset. If you want the
number of observations within your group you need to type:

marksample touse
qui count if `touse'
local Nby = r(N)

Hope this helps,
Maarten

On Fri, Aug 19, 2011 at 9:41 AM, 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:
>> *   http://www.stata.com/help.cgi?search
>> *   http://www.stata.com/support/statalist/faq
>> *   http://www.ats.ucla.edu/stat/stata/
>>
>
>
>
> --
> 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:
> *   http://www.stata.com/help.cgi?search
> *   http://www.stata.com/support/statalist/faq
> *   http://www.ats.ucla.edu/stat/stata/
>



-- 
--------------------------
Maarten L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen
Germany


http://www.maartenbuis.nl
--------------------------

*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/


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