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st: RE: Counting combinations of two vars within groups defined by third var


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: Counting combinations of two vars within groups defined by third var
Date   Thu, 10 Nov 2005 17:50:22 -0000

This has been discussed quite recently on the list. 

You can make progress is several different ways. 

One is to use -contract- with the -zero- option. 
The price is a reduced dataset. 

Another is to install -groups- from SSC. 

Both of those in some sense "populate" the 
cells of a cross-combination as you imagine. 

But you needn't do it that way, and just plain 
old official Stata will get you there. 

The best way I can think of at the moment is probably 
something like 

egen malegrad = sum(gender == 1 & grad == 1), by(school) 
label var malegrad "male graduates" 
tabdisp school, cell(malegrad) 

In a school with all females, gender is always 0, 
and so 

gender == 1 & grad == 1

is never true (meaning 1) and the sum calculated
is all zero. 

There's an FAQ on true and false in Stata that may
be useful background reading here. To repeat, the principle is 
that counting (male graduates, in this case) is 
just a special case of summation. 

By the way, you have to look carefully at the definition
of -egen, count()-, as it doesn't do what one might guess. 

Nick 
n.j.cox@durham.ac.uk 

Julie K. Rajaratnam
 
> I have three variables - school (grouping variable), gender 
> (binary), and
> grad (binary). What I want to calculate is for each school, 
> how many males graduated? 
> 
> I have done the following:
> 
> sort sch gender grad
> by sch: egen mg = count(lastsch) if gender==1 & grad==1
> by sch: egen malegrads = max(mg)
> drop mg
> 
> However, this only works if there are one or more male grads 
> in the school.
> I can't figure out how to get zero values to populate in 
> schools where the if condition is not satisfied.

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