st: Requirement(s) for a match merge

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

Dear All,
Is it true that "for a match merge to work, the identifier or identifiers 
must uniquely identify each observation"? I found this statement in sample 
lecture NC 101 (one of StataCorp's NetCourses), but I thought that this 
requirement (of the id uniquely identifying each observation) is often 
desirable, but not necessary in all cases. Any insights will be appreciated. 
Thank you.

Louis


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