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Re: st: Computing difference of counts within groups

From   Phil Schumm <>
Subject   Re: st: Computing difference of counts within groups
Date   Mon, 1 Nov 2010 11:09:20 -0500

On Nov 1, 2010, at 10:46 AM, Grealy, Patrick J wrote:
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

One way to replicate exactly what you have done is the following:

    egen fdiff = total((foreign*2)-1) if !mi(rep78), by(rep78)
    replace fdiff = abs(fdiff)

-- Phil

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