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MathGroup Archive 2005

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NonlinearFit-Logistic Function-CalcCenter 3

  • To: mathgroup at smc.vnet.net
  • Subject: [mg60498] NonlinearFit-Logistic Function-CalcCenter 3
  • From: Oddur Bjarnason <oddur.bjarnason at broadpark.no>
  • Date: Sun, 18 Sep 2005 01:15:53 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

I thank Bob Hanlon and Valeri Astanoff for their prompt reply and am following their suggestions.

Purely by chance I found a solution to the problem I had using CalcCenter 3 for nonlinear fitting of data to a logistic function.

By fitting 

data2={{-0.08,0.05},{0.96,-3.19},
      {1.93,-6.4},{2.98,-10.32},
      {3.97,-11.8},{5.92,-13.98},
      {7.88,-14.12},{11.85,-15.34},
      {15.79,-14.61},{19.7,-15.43},
      {23.67,-15.83}};

by NonlinearFit[data2, -a/(1 + b*Exp[-c*t]), t, {a, b, c}]

instead of

NonlinearFit[data2, a/(1 + b*Exp[-c*t]), t, {a, b, c}]

that is by changing the sign of a

I obtained the function

-14.917794621733984/(1 + 10.284628991516529*Exp[-1.0072061447043623*t]

Presumably this method can also be used in Mathematica.

I must admit that I have not as yet found out why this works.

Regards,

Oddur Bjarnason



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