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st: merge creates duplicates in master data

From   Will Hauser <>
Subject   st: merge creates duplicates in master data
Date   Sun, 25 Apr 2010 22:42:35 -0400

Hello all,

I am experiencing unexpected behavior in Stata 10 when using the merge command. I am matching two lists based on a series of string variables (first name, last name, initials) and one numeric region identifier. I have carefully cleaned the string variables of excess spaces and punctuation marks but they are inherently difficult to match as the name on one list may correspond to a nick name or abbreviation on the other (e.g. "WILLIAM" may correspond with "W" or "BILL"). My approach to this problem is to make multiple merges between the two lists each time using less information. For example, the first merge uses first name, last name, and region. The second uses first initial, last name, and region. The third just last name and region (and so on). Since the master data is inviolate subsequent mismatches should never overwrite earlier 'good' matches. I am using the update option but not the replace option. I am not using the unique option since the variables do not uniquely identify the cases in either the master or the using.

From what I can tell Stata is duplicating cases in the master dataset. The end result is 10 pairs of duplicate entries that appear identical in every way save for the _merge summary variable from the last merge. The summary variable indicates using agrees with master (3) for one of the duplicates and indicates that using does not agree with master for the other (5). There are no missing values in either list and I can see nothing special about the entries that are duplicated. I have used the duplicates command to verify that these duplicates are not present in the master data prior to merging.

I assume this is not a bug but is rather something about the merge command I am misunderstanding and that concerns me very much. I would be happy to provide the lists and the relevant portion of the do file if anyone is interested. The lists are public and are not unusually long (958 cases in the master and 593 cases in the using).

Thanks for your insight,

William Hauser
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