Problem with Fit
- To: mathgroup at smc.vnet.net
- Subject: [mg17304] Problem with Fit
- From: Richard Russell <rtr at rd.bbc.co.uk>
- Date: Fri, 30 Apr 1999 23:22:30 -0400
- Organization: British Broadcasting Corporation, UK
- Sender: owner-wri-mathgroup at wolfram.com
I have experienced what appears to be an underflow or overflow problem
when using Fit. I fitted a quadratic {1,x,x^2} to a set of 26 data
points where the x-value was in the range 0 to 25000 and the y-value
was in the range 0 to 40; the results were as expected. I then tried
fitting a cubic {1,x,x^2,x^3} and the results were much worse, with
a larger RMS error than the quadratic. Trying higher powers such as
{1,x,x^2,x^4} and {1,x,x^2,x^5} gave increasingly poor fits.
I then repeated the process but with all the x-values divided by 1000,
i.e. in the range 0 to 25. This time everything worked correctly and
the cubic fit was significantly better than the quadratic. There had
been no warnings or error messages with the original values, the only
indication of the problem being the very poor fit.
Is this a known problem? Is it reasonable to expect Mathematica to be
able to issue a warning that something is amiss? The version I am using
is Mathematica 2.2 for SPARC; is there a later one for this platform?
Richard.