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st: Count of unique cases by group

From   "Ben Hoen" <>
To   <>
Subject   st: Count of unique cases by group
Date   Fri, 6 Jan 2012 10:30:47 -0500

Happy New Year Statalisters!

I have been going in circles on a seemingly easy question and am hoping
someone can simplify this.

I want to count the number of unique cases within a group to generate a
summary table.  .collapse gets me part of the way there, but not all of the

sysuse auto, clear
g make2=word(make,1)
rename rep78 area
keep make2 area price
keep in 1/20
drop if area==.
sort area make2
order area make2

Using these records, I would like to produce a table with a statistic
representing the count of the unique groups of make2 in each area, divided
by the count of make2 in each area.  

i.e. pctcount=count of groups of make2 / count of make2

So the output table would look like this ideally (minus the "underlying
calculation" column):

area	pctcount	<underlying calculation>
2	0.6666		2/3
3	0.25		4/12
4	1		2/2
5	1		1/1

Thanks, in advance, for any thought you give this.



Ben Hoen
Principal Research Associate
Lawrence Berkeley National Laboratory
Office: 845-758-1896
Cell: 718-812-7589

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