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Re: st: Graph Quadratic Term


From   Dennis Kramer <dkramerii@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Graph Quadratic Term
Date   Tue, 19 Apr 2011 13:17:21 -0400

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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