```Date: Wed, 22 Dec 1999 23:47:35 -0800 Reply-To: Kattamuri Sarma Sender: "SAS(r) Discussion" From: Kattamuri Sarma Subject: Re: a question in regression Comments: To: Paige Miller Content-Type: text/plain; charset="iso-8859-1" In my comments I have assumed that the coefficients are statistically significant, the fit is good. In addition to these statistical tests, we should make tests to see whether the magnitudes are meaningful. Let me give an example from economics. Suppose you are fitting an equation to some economic data, your coefficients may be statistically significant, you may have an overall good fit. But the equation itself may not be consistent with economic theory. It happens all the time. You get statistically good fit, but meaningless from economic theory. I am sure it happens in other fields as well. For this you do additional tests, sensitivity tests, multiplier tests, etc. Examining the derivatives is one such test. It is not a statistical test , but a plausibility test from the point of economic theory. Kattamuri Sarma -----Original Message----- From: Paige Miller Newsgroups: bit.listserv.sas-l To: SAS-L@LISTSERV.UGA.EDU Date: Wednesday, December 22, 1999 5:02 AM Subject: Re: a question in regression >Kattamuri.Sarma@RESPONSEINSURANCE.COM wrote: >> >> Take the partial derivative of Y w.r.t X, which will be something like >> a +2bX, ( I am assuming >> that you have fitted Y = constant + aX + b*X**2 ) and see how this >> behaves >> at each plausible value of X. Only from the partial derivative >> evaluated at each x, can you tell whether you >> have a meaningful equation or not... > >I would have advised, since we are talking about a regression, which is >by nature empirical, that the derivatives aren't the only thing you >might want to consider, but also how well your fitted model predicts the >data. In fact, I would put a lot more emphasis on model fit than on the >derivatives. > >-- >Paige Miller >Eastman Kodak Company >paige.miller@kodak.com >"It's nothing until I call it!" -- Bill Klem, NL Umpire ```

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