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Re: polyfit weirdness

 From: Etienne Grossmann Subject: Re: polyfit weirdness Date: Tue, 11 Feb 2003 08:46:35 +0000 User-agent: Mutt/1.3.28i

```On Tue, Feb 11, 2003 at 02:08:37AM -0600, Scott Lamb wrote:
# I'm trying to use Octave (2.1.36, 2.1.44) for the first time. I want to
# put some data in, make a trendline, and make a graph with the raw data,
# trendline, trendline's formula, and correlation displayed, and attach
# them to a LaTeX document when I'm done. (The sort of thing I could do
# easily in Excel, but I want to learn a new open-source tool.)
#
# So first I'm calling polyfit to get the trendline:
#
#     l1 = 1e-9 * [ 578.45 545.88 435.87 404.71 365.26 ]
#     v1 =        [   0.66   0.79   1.40   1.58   1.92 ]
#     f1 = 299792458 ./ l1

f1 =

5.1827e+14   5.4919e+14   6.8780e+14   7.4076e+14   8.2076e+14

A problem with order of magnitude of f1 and floating-point
limitations?

octave:63> f1 /= 1e14;
octave:64>      [p1,cv1] = polyfit(f1,v1,1)'
p1 =

0.41679
-1.49481

cv1 =

0.66528  0.79417  1.37189  1.59260  1.92606

Is that more like what you expected?

Hth,

Etienne

#     [p1,cv1] = polyfit(f1,v1,1)
#
# ...and it doesn't return what I'd expect at all:
#
#     p1 =
#
#        1.9796e-15
#        2.8980e-30
#
#     cv1 =
#
#       1.0260  1.0872  1.3616  1.4664  1.6248
#
# cv1 should hold its computed (near-identical to v1) values, right? But
# they're not even close. Doing this in Excel gives a slope of 4.17e-15
# and R^2 = .999. So these definitely aren't right. What am I doing
# incorrectly?
#
# Thanks,
# Scott Lamb
#
#
#
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--
Etienne Grossmann ------ http://www.isr.ist.utl.pt/~etienne

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```