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RE: st: Categorizing HIV status using a series of string variables

From   Howard Lempel <>
To   "" <>
Subject   RE: st: Categorizing HIV status using a series of string variables
Date   Mon, 24 Nov 2008 21:17:17 -0500


I haven't used regular expressions in a bit, so someone should correct me if I'm wrong, but I think the problem you mention would be solved by replacing Tom's first line of code with:

gen group = (regexm(HIV, "N[\.I]*P"))

The expression in the brackets tells Stata to ignore "."s and "I"s when it looks for an N followed by a P.  For more info on using regular expressions, check here:  -help regexm- will also be useful.

Also, I don't know if you have any cases where someone is indeterminate or missing in every period.  If you have any such cases, I think Tom's code will code those as group 3, which does not seem appropriate. You may want to add a line of code as follows:

replace group = 4 if !regexm(HIV, "N") & !regexm(HIV, "P")

You will also want value labels for your variable.  The following code should -label- your group variable.

lab define grouplab 1 "incident seroconverter" 2 "prevalent positive" ///
        3 "consistently seronegative" 4 "missing/indeterminate"

lab val group grouplab

Lastly, I'd like to warn you that I've had some trouble with the way that Stata's -regexm- function deals with missing values.  If you have any truly missing values (i.e. ""), I would carefully check to make sure that the -regexm- function is dealing with them in the right way.  See this thread for the problem I had:

Hope this helps

Howie Lempel
Research Assistant
The Brookings Institution | Economic Studies

1775 Massachusetts Ave NW | Washington DC 20036 | p: (202) 238-3576

-----Original Message-----
From: [] On Behalf Of Polis, Chelsea B.
Sent: Monday, November 24, 2008 8:49 PM
Subject: RE: st: Categorizing HIV status using a series of string variables

Many thanks, Tom and Howie!

Tom: Your solution worked beautifully, except for one tiny thing.  I got a few people who weren't assigned to one of the three groups, and their codes all had one thing in "indeterminate" test between their negative and positive tests:


I can very easily just recode these people to be incident seroconverters, but I wonder if there is an easy fix for the code that would do this automatically?

Howie: Many thanks for the explanation...that clears up a lot of my confusion!

BTW: my apologies if this post ends up in the wrong spot - I'm still trying to understand how to reply to individual postings when I receive Statalist in digest form...I'm hoping that slapping a "RE:" in front of the subject line I wish to respond to will allow me to do that, but I couldn't find information in the FAQ on specifically how to do this.


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