At 4:21 AM -0400 7/1/00, Donald Burrill wrote:
As I vaguely recall, I found this years ago in Snedecor and Cochran.
...
> Using linear through cubic predictors almost works, but has the
>interesting defect of predicting a negative weight for day 6; since one
>clearly doesn't want to stop at quadratic, the optimal polynomial is
>quartic (quintic and sextic contributions being negligible). Some
>students would prefer the polynomial function because the R-sq is larger
>than for the exponential (although the R-sq values are not strictly
>comparable, being based on different dependent variables);
I'm not real comfortable with a polynomial model that takes nearly
half the available degrees of freedom and offers no theoretical
motivation. I'd rather fit log(wt) on day. If you don't like the
trivial wiggle left in the residuals, throw in the quartic term, but
I don't really like that. I'd prefer to excuse the first day as
typically unusual in such series and then inquire whether there is
anything special about the period around day 11.
-- Paul
--
Paul F. Velleman
Cornell University Data Description, Inc.
358 Ives Hall Box 4555
Ithaca, NY 14853 Ithaca, NY 14852-4555
(607) 255-4411 (607) 257-1000
(607) 255-8484 fax (607) 257-4146 fax
===========================================================================
This list is open to everyone. Occasionally, less thoughtful
people send inappropriate messages. Please DO NOT COMPLAIN TO
THE POSTMASTER about these messages because the postmaster has no
way of controlling them, and excessive complaints will result in
termination of the list.
For information about this list, including information about the
problem of inappropriate messages and information about how to
unsubscribe, please see the web page at
http://jse.stat.ncsu.edu/
===========================================================================
