Date:         Tue, 21 Dec 1999 15:25:12 GMT
Reply-To:     dkb@CIX.COMPULINK.CO.UK
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         dkb@CIX.COMPULINK.CO.UK
Organization: CIX - Compulink Information eXchange
Subject:      Re: a question in regression

Paige Miller posts:

> > It is counterintuitive that y is positively related to x and then
negatively
> > related to x^2.

> Not knowing the problem, I see no a
> priori reason why the derivative must be positive whenever x > 0.
> Letting b be < 0 allows negative derivatives of your quadratic function,
> which seems quite possible to me.

Height y of a projectile at time x is a very simple example of such a
function; as you say, not knowing the problem, negative b can't be ruled
out.
.