|Date: ||Tue, 21 Dec 1999 10:01:15 -0500|
|Reply-To: ||Paige Miller <paige.miller@KODAK.COM>|
|Sender: ||"SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>|
|From: ||Paige Miller <paige.miller@KODAK.COM>|
|Organization: ||Eastman Kodak Company|
|Subject: ||Re: a question in regression|
|Content-Type: ||text/plain; charset=us-ascii|
Hongjie Wang wrote:
> Dear friends:
> Recently, I saw a regression mode y=a x + b (x^2) + some other stuff.
> where a and b are two numbers of opposite signs. Does that seem normal?
> It is counterintuitive that y is positively related to x and then negatively
> related to x^2.
> But then, it is multivariate analysis. Any insights will be appreciated.
Y is a quadratic function of x. If the value of b is positive, as you
think it should be, then the derivative of the quadratic function will
be positive whenever x is positive. 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.
By the way, I do not consider this multivariate analysis. The
terminology I am familiar with does not call regression with a single Y
Eastman Kodak Company
"It's nothing until I call it!" -- Bill Klem, NL Umpire