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MathGroup Archive 2008

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FindFit issue

  • To: mathgroup at smc.vnet.net
  • Subject: [mg86395] FindFit issue
  • From: t.dubaj at gmail.com
  • Date: Mon, 10 Mar 2008 02:04:34 -0500 (EST)

Hi,
I am trying to fit experimental data

d = {{1000, 44.576*10^(-3)},
  {600, 49.546*10^(-3)},
  {800, 46.757*10^(-3)},
  {400, 55.023*10^(-3)},
  {200, 61.209*10^(-3)},
  {10, 71.584*10^(-3)},
  {100, 65.367*10^(-3)}}

and model is:

y = 0.072214 - a*Log[10, 1+b*x]

but Mathematica after typing

FindFit[d, 0.072214 - a*Log[10, 1 + b*x], {a, b}, x]

return

FindFit::nrlnum: The function value {-0.0226973-0.0208168 \
\[ImaginaryI],<<5>>,-0.0282009-<<21>> \[ImaginaryI]} is not a list of
\
real numbers with dimensions {7} at {a,b} = {0.0152574,-1.99204}.

{a -> 0.0152574, b -> -1.99204}

(especialy value of "b" is really awful - negative number in Log)

But I know, there is  Fit (calculated in Scientist) with coefficients
a = 0.033706
b = 0.0057934

Can someone explain what I'm doing wrong?

Best regards,

T.D.




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