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Re: st: Post codes to locality


From   Gordon Hughes <G.A.Hughes@ed.ac.uk>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Post codes to locality
Date   Tue, 24 Apr 2012 11:32:08 +0100

To clarify the point that is implicit in other advice:
A. This is a warning message not an error. The situation in which one dataset contains multiple cases with the same merge id (postcode) is routine. It gives rise to what is called "one-to-many" (1:m or m:1) matches - i.e. the same LSOA (and other data) is associated with each case with the same postcode. B. There would be a problem if you get the warning message that both datasets contain multiple occurrences of the same merge id. This leads to a "many-to-many" (m:m) match: the results are predictable but probably not what you intended. The details are given in the manual. C. The merge syntax introduced in Version 11 makes this explicit - i.e. either -merge 1:m ...- or -merge m:1 ...- specify a "one-to-many" merge. Many of us are used to the older syntax which does not include <1:m> or the like, but -merge- will generate the same results provided that you are not using a m:m merge. You may think of the new syntax as way of checking that you understand your data and will get what you want.

Gordon Hughes
g.a.hughes@ed.ac.uk
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