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