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Re: st: simple question


From   Trudy Ann Cameron <cameron@uoregon.edu>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: simple question
Date   Wed, 02 Jun 2004 12:42:54 -0700

Suzy,

You can create a dummy variable for "have_variable" that takes a value of 1
if the data are non-missing and zero otherwise.  Then you can interact this
variable with the continuous variable.  The slope that you estimate on the
interaction with the continuous variable is a slope CONDITIONAL on the
availability of data for that variable.  The coefficient on the dummy
variable is the difference in the expected value of Y according to whether
the data on your continuous variable is missing or not.

There is not much you can do if the continuous variable is your dependent
variable, unless you know that 99999 is a top-coded variable, or something
like that.

The alternative is to estimate a selection correction model using the
"have_variable" dummy as the selection variable, and to correct the "main"
regression for any selection bias.

Trudy

At 12:17 PM 6/2/2004, you wrote:
>Hi, I have continuous variables that are coded 99999 or 77777 for unknown
>or refused to answer. How do I handle this? I don't want to treat them as
>missing, if possible.
>Thank you.
>Suzy
>
>
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Trudy Ann Cameron
R.F. Mikesell Professor of Environmental
            and Resource Economics
Department of Economics, 430 PLC
1285 University of Oregon
Eugene, OR  97403-1285

phone:  (541) 346-1242
FAX:     (541) 346-1243
email:    cameron@uoregon.edu


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