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MathGroup Archive 2004

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Re: 'NonlinearFit` confusion

  • To: mathgroup at smc.vnet.net
  • Subject: [mg49878] Re: 'NonlinearFit` confusion
  • From: Paul Abbott <paul at physics.uwa.edu.au>
  • Date: Thu, 5 Aug 2004 09:20:41 -0400 (EDT)
  • Organization: The University of Western Australia
  • References: <ceqt91$jr7$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

In article <ceqt91$jr7$1 at smc.vnet.net>, klingot at yahoo.com (Klingot) 
wrote:

> I'm trying to fit a sinusoidal function to my data using
> 'NonlinearFit' but it's exhibiting rather odd behaviour. Please see my
> examples below:
> 
> EXAMPLE (1).
>    
> **** As a test, I created a list of data from a function of the form y
> = 6 + 2Sin[3 + x] with:
> 
> datay = Table [6 + 2Sin[3 + x], {x, -10Pi, 10Pi, 0.05}];
> datax = Table [x, {x, -10Pi, 10Pi, 0.05}];
> data = Table[{datax[[i]], datay[[i]]}, {i, 1, Length[datay]}];

You can generate this data directly using

  data = Table[{x,6 + 2Sin[3 + x]}, {x, -10Pi, 10Pi, 0.05}]

> **** Then tested to see whether NonlinearFit would correctly deduce
> the equations parameters with:
> 
> NonlinearFit[data, c + a Sin[d + e x], x, {a, c, d,e}]
> 
> **** As expected, it gave me '6.+ 2. Sin[3. + 1. x]' ... exactly as
> one would expect :)
> 
> 
> EXAMPLE (2).
> 
> **** Second test, I modified the equation by multiplying x by 5, ie. 
> y = 6 + 2Sin[3 + 5x]:
> 
> datay = Table [6 + 2Sin[3 + 5 x], {x, -10Pi, 10Pi, 0.05}];
> datax = Table [x, {x, -10Pi, 10Pi, 0.05}];
> data = Table[{datax[[i]], datay[[i]]}, {i, 1, Length[datay]}];

After re-generating your data,

  data = Table[{x,6 + 2Sin[3 + 5 x]}, {x, -10Pi, 10Pi, 0.05}]

have a look at it:

  ListPlot[data]

I think you will see what the problem is.

> ***** and applied the NonlinearFit as before:
> 
> NonlinearFit[data, c + a Sin[d + e x], x, {a, c, d,e}]
> 
> ***** but this time I get a wildly innacurate result:    6.000165 +
> 0.025086 Sin[0.0080308 - 0.247967 x]
> 
> Specifically, the parameters 'a', 'd' and 'e' are all completely in
> error by orders of magnitude.
> 
> I tried extending the range of the data to include more cycles of the
> sinusoid, thereby making it more continuous/monotonic but that made no
> difference.

In 5.0 you can use FindFit instead of NonlinearFit. Note that you may 
still have to provide a reasonable initial guess for the parameters to 
obtain convergence.

Cheers,
Paul

-- 
Paul Abbott                                   Phone: +61 8 9380 2734
School of Physics, M013                         Fax: +61 8 9380 1014
The University of Western Australia      (CRICOS Provider No 00126G)         
35 Stirling Highway
Crawley WA 6009                      mailto:paul at physics.uwa.edu.au 
AUSTRALIA                            http://physics.uwa.edu.au/~paul


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