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st: RE: RE: -prgen- with a quadratic function


From   "Scott Merryman" <smerryman@kc.rr.com>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: RE: -prgen- with a quadratic function
Date   Tue, 15 Mar 2005 19:33:25 -0600

Two other possibilities.

1. Use -probpred-  with the poly(#) option allows for variable of interest
to be entered in the model as a polynomial.

2. Construct a data set with the values you want before running -predict-

For example:

sysuse auto, clear
gen mpg2 = mpg^2
logit foreign price mpg mpg2 weight gear_ratio

foreach var of varlist price weight gear_ratio {
 qui sum `var', meanonly
 local ave_`var' = r(mean)
}

preserve
drop _all
*Predictions from 10 to 60
set obs 51
gen mpg = _n +  9
gen mpg2 = mpg^2

foreach var of newlist price weight gear_ratio {
 gen `var' = `ave_`var''
}

predict pred,p
twoway line pred mpg
restore

Hope this helps,
Scott



> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-
> statalist@hsphsun2.harvard.edu] On Behalf Of Nick Cox
> Sent: Tuesday, March 15, 2005 4:04 PM
> To: statalist@hsphsun2.harvard.edu
> Subject: st: RE: -prgen- with a quadratic function
> 
> I don't think Scott Long is a member of Statalist.
> 
> No matter: your problem should yield to -adjust-
> followed by an appropriate graph.
> 
> Nick
> n.j.cox@durham.ac.uk
> 
> Mariano Sana
> 
> > 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?
> 
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