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st: Merging longitudinal data set

From   Andreas Jensen <>
Subject   st: Merging longitudinal data set
Date   Fri, 16 Jul 2010 23:59:40 +0200

Hi Statalist.

I'm a new Stata user (previously only used R) and I'm working on a
project involving a longitudinal data set. There are two waves each
contained in its own data file and there is a common ID variable which
is consistent among the waves. I'm trying to wrap my head around the
merge command and I'm sure this is a fairly basic question. I,
however, would appreciate some confirmation on that what I'm doing is
hopefully correct.

What I'm troubled about is that there are people from wave 1 that has
dropped out when wave 2 was conducted (their ID does not exist in the
wave 2 data file), and there has been added additional people in wave
2 that aren't present in wave 1 (their ID does not exist in the wave 1
data file).

I have sorted each data file according to the ID variable and then
executed a merge 1:1 on the ID with wave 1 as master. I get the
following output.

    Result                           # of obs.
    not matched                        28,046
        from master                    12,373  (_merge==1)
        from using                     15,673  (_merge==2)

    matched                            18,742  (_merge==3)

So assuming that my command is correct, is it then true that there are
18742 individuals in both waves, 12373 individuals which has dropped
out after wave 1 and 15673 individuals that have been added in wave 2?

Thanks in advance,
Andreas Jensen
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