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Re: st: Matching in STATA

From   Svend Juul <SJ@SOCI.AU.DK>
To   <>
Subject   Re: st: Matching in STATA
Date   Fri, 20 Jun 2008 13:25:09 +0200

Henry wrote:

I would like to carry out some matching for a case-control study using
STATA but its proving to be a bit challenging to me. I have checked
from achieves but a query close to mine on statlist was not answered
in 2004. Could there be a way of matching cases to controls within a
range of values say for age, a 40yr old case-patient can be matched to
either a 38 or 39 or 40 or 41 or 42yr old control-patient? I have used
the -merge- command to merge two datasets by sex and age of patients
but it only works for 40yr old case matching a 40yr old control. For
this case am still interested in a 1-1  matching but what if I extend
this to a 1:n match?  I want to have something of this sort:

case-patient  case-age  sex  control-patient  control-age
        00b7        35    1             00YP           35
        00b7        35    1             0XC1           33
        00b7        35    1             0001           36


I get the impression that data have already been collected, and that
the purpose of matching is to facilitate analysis (at the cost of 
dropping some of the control observations). Actually, matching
complicates rather than facilitates analysis in case-control studies; 
at least you need to use conditional logistic regression (or -mcc-) to 
analyse correctly. So, if my impression is right, the recommendation 
is to analyse with -logistic- (or -cc-) including the potential 
confounders of interest, but avoiding to match and to remove any of 
the control observations. A variable like age could be grouped, e.g.,
in five-year groups.

Anyway, if you want or need to match, the usual way is to categorize 
a variable in, e.g., five year groups: 30-34, 35-39, etc. This is
more handy, and it also facilitates reporting the results (you can
stratify by age group).

Hope this helps


Svend Juul
Institut for Folkesundhed, Afdeling for Epidemiologi
(Institute of Public Health, Department of Epidemiology)
Vennelyst Boulevard 6
DK-8000  Aarhus C, Denmark
Phone:  +45 8942 6090
Home:   +45 8693 7796

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