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Groups > comp.soft-sys.math.mathematica > #1421

Re: NonlinearFit for specific data

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Dear Alex,

positions of arguments of parameters {a, b, c}
and variable, x, in your code is exchanged

(try this:
data2 = {{0, 773.59}, {30, 2351.12}, {60, 4205.8}, {90,
     4222.69}, {120, 4222.69}, {150, 704.328}, {180, 1388.28}, {210,
     1388.28}, {240, 1821.23}, {270, 1821.23}, {300, 754.109}, {330,
     773.59}, {360, 773.59}};
ff = FindFit[data2, Exp[a + b x + c*x^2], {a, b, c}, x]
Show[{ListPlot[data2], Plot[Exp[a + b x + c*x^2]
  /. ff, {x, 0, 400}]}]
)

However, its correction does not help.

I guess, that the data you have does not look like the curve you try to fit to it to the extent
that Mathematica is beaten out of a reason.

Probably a better idea would be to make the procedure "manually". My suggestion would be like follows:

Manipulate[

  res = Total[
    Table[(data2[[i, 2]] - a*Exp[b*x + c*x^2] /.
       x ->  data2[[i, 1]])^2, {i, 1, Length[data2]}]];
  Show[{ListPlot[data2],
    Plot[a*Exp[b *x + c*x^2], {x, 0, 400}]
    },
   Epilog ->
    Inset[Row[{Style["Residual=", 14], Style[res, 14]}],
     Scaled[{0.7, 0.8}]]
   ],

  {{a, 3000}, 0, 4000}, {b, -0.001, 0}, {c, -0.00001, 0}]

You can then play with limits for a, b and c to get a better curve. However, the residual is so huge,
that I believe that fitting of this data is in general a bad idea.

Have fun, Alexei


Dear Group,
I have the following data :
data2 = {  {0, 773.59},{30, 2351.12},{60, 4205.8}, {90, 4222.69},{120,
4222.69},{150, 704.328},
{180, 1388.28},{210, 1388.28},{240, 1821.23},{270, 1821.23},{300,
754.109},{330, 773.59},{360, 773.59}};
I want the following curve fit :
  NonlinearFit[data2, Exp[ a + b x + c x^2], x, {a, b, c}]
Unfortunatelly I get some "FindFit::sszero
FindMinimum::sszero" message, and the given result is ackward.
I also tried FindFit and NonlinearRegress without success.
Any comment would be very welcome.
Alex


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Re: NonlinearFit for specific data Alexei Boulbitch <alexei.boulbitch@iee.lu> - 2011-04-02 07:53 +0000

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