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st: RE: Computing difference of counts within groups

From   Nick Cox <>
To   "''" <>
Subject   st: RE: Computing difference of counts within groups
Date   Mon, 1 Nov 2010 16:14:04 +0000

No looping needed at all. This yields to -by-. 

sysuse auto, clear 
egen nforeign = total(foreign == 1), by(rep78) 
egen ndomestic = total(foreign == 0), by(rep78) 
gen diff = nforeign - nodomestic 


-egen- here is optional, as 

bysort rep78: gen nforeign = sum(foreign == 1) 
by rep78: replace nforeign = nforeign[_N] 
by rep78: gen ndomestic = sum(foreign == 0)
by rep78: replace ndomestic = ndomestic[_N]

gets you to the same place. 


Grealy, Patrick J

I thought there was a simple way to do what I am attempting using the egen command but I have not gotten that to work right. Can someone make another suggestion? I am trying to check if the difference between two groups within another group exceeds a certain value. For example, using the auto dataset, within the five rep78 groups I would like to calculate the difference between Domestic and Foreign and check if it exceeds some limit, say 7.

This code below does what I want but I was hoping to find something simpler:

sysuse auto,clear
gen fdiff = .
foreach ix_rep78 of numlist 1/5 {
quietly count if foreign==0 & rep78==`ix_rep78'
local foreign0 `r(N)'
quietly count if foreign==1 & rep78==`ix_rep78'
local foreign1 `r(N)'
replace fdiff = abs(`foreign0' - `foreign1') if rep78==`ix_rep78'
tab rep78 foreign if fdiff > 7

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