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RE: st: Graph Quadratic Term


From   "Fernando Rios Avila" <f.rios.a@gmail.com>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: Graph Quadratic Term
Date   Tue, 19 Apr 2011 13:34:16 -0400

Dennis
Some additional suggestions to previous comments,
First, Make sure you are considering all the other dependent variables in
your regression constant for the graph, So it only depends on the linear and
quadratic term
Second, Perhaps if you try making the graph on the linear prediction first,
and then on the probability prediction, can give you a better idea of the
shape.
Third, Be sure the range you?re your quadratic term contains the inflextion
point.
Fernando

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Dennis Kramer
Sent: Tuesday, April 19, 2011 1:17 PM
To: statalist@hsphsun2.harvard.edu
Subject: Re: st: Graph Quadratic Term

Steve, it is perfectly linear. However, when estimate points on the point it
should show a large bell-shaped curve.

On Tue, Apr 19, 2011 at 12:48 PM, Steven Samuels <sjsamuels@gmail.com>
wrote:
>
> Dennis,
>
> Quadratic trends can look linear. If you print out the graph and lay a
ruler over the extreme points, what do you see?
>
>
> Steve
> sjsamuels@gmail.com
>
>
> On Apr 18, 2011, at 1:00 PM, Nick Cox wrote:
>
> I don't know what you mean by "does not plot correctly" and in any 
> case you give very little code.
>
> -predict- is a command, not a function.
>
> This example may help
>
> sysuse auto
> gen headroomsq = headroom^2
> logit foreign headroom headroomsq
> predict predict, pr
> scatter foreign headroom || mspline predict headroom
>
> A quadratic on the logit scale naturally translates to a bell-shape on 
> the original scale.
>
> Nick
>
> On Mon, Apr 18, 2011 at 5:36 PM, Dennis Kramer <dkramerii@gmail.com>
wrote:
>
> > I am having trouble graphing the quadratic relationship between my 
> > outcome dependent and independent variable. I am currently modeling 
> > a
> > -logit- function with my outcome being 0/1 for passing a 
> > standardized test. When I run my --logit-- with the sq term in the 
> > model I get a significant quadratic function; however, when I use 
> > the -- predict, pr
> > -- function to plot for predictions the quadratic function does not 
> > plot correctly... could you provide some insight.
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