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Re: st: _N in by-groups

From   Maarten Buis <>
Subject   Re: st: _N in by-groups
Date   Fri, 19 Aug 2011 09:07:19 +0200

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 L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen
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