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st: expanding obs, then changing values based on that


From   Michael Costello <michaelavcostello@gmail.com>
To   statalist <statalist@hsphsun2.harvard.edu>
Subject   st: expanding obs, then changing values based on that
Date   Fri, 20 May 2011 23:44:00 -0400

Statlisters,
I have a dataset with a variable that asks teachers which section they
teach.  The options are sections a though e, and more than one answer
is allowed.  Thus:
teacher_id    var_a   var_b   var_c   var_d   var_e
1             1       0       0       0       0
2             1       1       1       0       0
3             1       0       0       1       1
4             1       0       0       1       0
I need to duplicate any observation that has a 1 for more than one of
var_a-var_e.  So I did:
egen var_total=rowtotal(var_a-var_e)
expand var_total
sort teacher_id
Now, for teacher #2 above for example, I need to assign one (and only
one?) of her three observations var_a=1, the second var_b=1 and the
third var_c=1.  This way I can create a new variable, "section" and
have observation for each section.  It would look like this in the
end:
teacher_id  section
1           a
2           a
2           b
2           c
3           a
3           d
3           e
4           a
4           d
I tried to create a concatenated variable so that I could assign "a"
if the first number in the concatenated string (from the first table
above) ==1, "b" if the second number were ==1, etc.  The problem is
that I cannot figure out how to only change the second observation
within one teach_id number.  Stated differently, it's changing all,
not cycling though to the next observation.
I'm not sure if this is making sense, but if you have ideas, please let me know.
-Michael

--
Michael Costello
Mathematics & Statistics Teacher, Bethesda-Chevy Chase High School
Intern, RTI International
202-246-1627
"To call in the statistician after the experiment is done may be no
more than asking him to perform a post-mortem examination: he may be
able to say what the experiment died of."  -Sir Ronald Aylmer Fisher,
FRS

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