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st: -prgen- with a quadratic function


From   Mariano Sana <msana@lsu.edu>
To   statalist@hsphsun2.harvard.edu
Subject   st: -prgen- with a quadratic function
Date   Tue, 15 Mar 2005 12:56:29 -0600




Hi Statalisters.  Someone experienced with ?prgen- might be able to answer
this.

I ran a logit model that has, among its predictor variables, AGE and AGESQ.
The coefficient for AGE is positive and the coefficient for AGESQ is
negative.  The turning point is at age 79 or so.  I?ve been trying to
generate a graph that shows how predicted probabilities vary by age for a
few groups in my analysis.  I used the ?prgen- command from the Spostado
package.

The graph that I get doesn?t make sense. The probability increases nonstop
and becomes asymptotic to 1 at age 90 or so.

I realize that the problem is that ?prgen- cannot vary the value of AGESQ
as the value of AGE varies.  The default for AGESQ is the mean of AGESQ,
unless I specify another value.  Obviously, I need AGESQ to be the square
of AGE as AGE varies.

By now I think I have no other solution than running ?prvalue- (also from
the Spostado package) multiple times to get the probabilities I need for
the graph. Or is there any other way? Is this a question for Scott Long?

Thanks in advance!

Mariano Sana

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