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Re: nonlinear fit question

  • To: mathgroup at
  • Subject: [mg60856] Re: nonlinear fit question
  • From: "Ray Koopman" <koopman at>
  • Date: Fri, 30 Sep 2005 03:57:41 -0400 (EDT)
  • References: <dhgepn$1rd$>
  • Sender: owner-wri-mathgroup at

Kevin Towles wrote:
> I am trying to do a nonlinear fit using a model I have developed.  My
> problem is that my model is discontinuous and I only want to allow
> the NonlinearFit function to search a range for the value of one of
> the parameters. The actual implementation would be something like:
> NonlinearFit[data,Model[xVar,yParam],xVar,yParam]
> but I want yParam to be confined to a range as: ymin <= yParam <= ymax
> I tried using NonlinearFit[data,Model[xVar,yParam],xVar,{yParam,
> {ymin,ymax}}] but the function is obviously searching outside this
> range.  I know there is a good fit in the range that I am specifying,
> so I don't know why it insists on searching outside the start
> values.  Is there a way to confine my yParam?
> Any help is greatly appreciated.
> Kevin Towles

If you don't expect the optimizing value of yParam
to be at either boundary then try fitting tParam,
with yParam = ymin + (ymax-ymin)/(1+Exp[-tParam]).

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