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Re: FindFit issue

  • To: mathgroup at smc.vnet.net
  • Subject: [mg86408] Re: FindFit issue
  • From: dh <dh at metrohm.ch>
  • Date: Tue, 11 Mar 2008 02:54:17 -0500 (EST)
  • References: <fr2mhj$o5h$1@smc.vnet.net>


Hi,

if FindFit does not have any initial values to start its search, it must 

  make an initial guess. Such a guess may result in a complex function 

value. To prevent this, you must specify a reasonable starting value.

hope this helps, Daniel



t.dubaj at gmail.com wrote:

> 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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