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.

**Follow-Ups**:**Re: FindFit issue***From:*Darren Glosemeyer <darreng@wolfram.com>