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Re: Problems getting CURVEFIT to work [message #32849 is a reply to message #32848]

Wed, 13 November 2002 07:18

Craig Markwardt
Messages: 1869
Registered: November 1996

Senior Member

Jonathan Greenberg <greenberg@ucdavis.edu> writes:
> On 11/12/02 5:14 PM, in article onk7jiuv7e.fsf@cow.physics.wisc.edu, "Craig
> Markwardt" <craigmnet@cow.physics.wisc.edu> wrote:
>> Jonathan Greenberg <greenberg@ucdavis.edu> writes:
>>> Hi there, I'm trying to use CURVEFIT to fit data to a decay function of the
>>> form:
>>> f(x) = a(1-e^(bx))+c
>>
>> Problem 1. Your parameters A and C are very highly (anti) correlated
>> with each other. It would be better to recast as A*EXP(B*X) + C.
>
> The problem is I need a curve that starts low and asymptotes higher -- I
> have a reason to believe it will asymptote at some maximum, hence the
> a(1-e^(bx))+c --> given this, how can I get this to work?

A*EXP(B*X) + D will suit you just fine. It's the same function after
all, with (D=C+A). The fit you were doing had C and A almost
perfectly anti-correlated, so combining the two will relieve some of
that problem.

Craig

--
------------------------------------------------------------ --------------
Craig B. Markwardt, Ph.D. EMAIL: craigmnet@cow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
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[Message index]

		Re: Problems getting CURVEFIT to work By: James Kuyper on Wed, 13 November 2002 07:41
		Re: Problems getting CURVEFIT to work By: Craig Markwardt on Wed, 13 November 2002 07:18
		Re: Problems getting CURVEFIT to work By: Jonathan Greenberg on Tue, 12 November 2002 23:43
		Re: Problems getting CURVEFIT to work By: Craig Markwardt on Tue, 12 November 2002 17:14
		Re: Problems getting CURVEFIT to work By: Jonathan Greenberg on Fri, 15 November 2002 10:35

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