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MathGroup Archive 2005

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A Problem with the NonlinearFit?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg61757] A Problem with the NonlinearFit?
  • From: "Axel Ligon" <ligonap at web.de>
  • Date: Fri, 28 Oct 2005 03:25:41 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Deas Sirs,

concerning the NonlinearFit procedure I have a problem. I used the 
following inputs:

A)
*Raw datas {x, y}*
dat1={{0,319.9},{1,196.7},{3,140.2},{7,99.0},{14,56.1},{31,32.8},{90,1.9}}
FindFit[dat1,a*Exp[-b*x],{a,b},x]

a ->280.376, b->0.175479

<<Statistics`NonlinearFit`
f=NonlinearFit[dat1,a*Exp[-b*x],x,{a,b}]

280.376 * Exp(-0.175479x)

B)
*Convert datas {x, ln[y]}*
dat2={{0,5.7680},{1,5.2817},{3,4.9430},{7,4.5951},{14,4.0271},{31,3.4904},{90, 0.6419}}
FindFit[dat2,-b*x+a,{a,b},x]

a->5.18756, b->0.0518199

<<Statistics`NonlinearFit`
f=NonlinearFit[dat2,-b*x+a,x,{a,b}]

5.18756 - 0.0518199 x

The difference between A) and B) is only the conversion of y to ln[y] 
and the equation y = a * exp(-b*x) to ln[y] = ln[a] - b*x. From this 
it follows that a = exp[ln[a]].
But I get in A) 280.376 / -0.175479 and in B) 179.031 / -0.05182 !!
Why do I get these error?? If I use Excel, I don't get these errors.
Is it an error in the programming of Mathematica, or not???

I'd be glad if you could help me.

With best regards

Axel Ligon



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