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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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