st: Making sure identifiers are unique

["louis boakye-yiadom" <louisby@hotmail.com>]

Dear all,
I've been trying to determine the identifiers of a data set, and to ensure 
they're unique. Suspecting the variables, "region" and "district" are the 
identifiers, I gave the commands below, and got the output shown:
. sort region district
. by region district: assert _N==1
62 contradictions in 97 by-groups
assertion is false
r(9);

Owing to the fact that I'm more interested in the "district"-level data, I 
wanted to know whether a collapsed version of the data will have unique 
identifiers. I therefore gave the following set of commands and got the 
results shown:
. gen x=1
. collapse (count) x, by (region district)
. sort region district
. by region district: assert _N==1

My question is: What can account for the collaped data being uniquely 
identified by "region" and "district", whilst the original data are not? I'm 
using version 8.2.

Many thanks,
Louis


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