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Re: IDL - EXP fitting function [message #65895 is a reply to message #65886]

Fri, 27 March 2009 07:03

Vince Hradil
Messages: 574
Registered: December 1999

Senior Member

On Mar 27, 8:50 am, Paolo <pgri...@gmail.com> wrote:
> Vince Hradil wrote:
>> On Mar 27, 8:27 am, Paolo <pgri...@gmail.com> wrote:
>>> Vince Hradil wrote:
>>>> On Mar 26, 5:55 pm, Christopher Thom <ct...@oddjob.uchicago.edu>
>>>> wrote:
>>>> > Quoth glen_a...@hotmail.com:
>
>>>> > > On Mar 26, 5:12 pm, David Fanning <n...@dfanning.com> wrote:
>>>> > > > glen_a...@hotmail.com writes:
>>>> > > > > Greetings everyone! My first post! I have some data x, y, that i would
>>>> > > > > like to fit to a fitting function of the kind yfit = EXP(a+ b*x).
>>>> > > > > where a and b are constants which i would like found. Any ideas on how
>>>> > > > > to do this?
>
>>>> > > > ab = LinFit(x, y)
>>>> > > > a = ab[0]
>>>> > > > b = ab[1]
>
>>>> > > > Cheers,
>
>>>> > > > David
>>>> > > > --
>>>> > > > David Fanning, Ph.D.
>>>> > > > Fanning Software Consulting, Inc.
>>>> > > > Coyote's Guide to IDL Programming:http://www.dfanning.com/
>>>> > > > Sepore ma de ni thui. ("Perhaps thou speakest truth.")
>
>>>> > > Thanks for getting back to me David,
>
>>>> > > Does the linfit function work when i would like my data to be fitted to
>>>> > > an EXP(a + bx) function? I didn't think that a linear function would be
>>>> > > correct when considering the EXP? Or am i getting confused going from
>>>> > > real space to log space!
>
>>>> > No, linfit() fits a linear model of the form y = A + B*x, so it will not
>>>> > "just work". why don't you just fit a linear model in logspace?
>
>>>> > res = linfit(x, alog(yfit))
>>>> > a = res[0]
>>>> > b = res[1]
>
>>>> > cheers
>>>> > chris
>
>>>> I'll second that. This is really a linear problem, so no need to
>>>> solve the non-linear equation.
>
>>> I disagree. If you have negative measurements, or positive
>>> but very small measurements, you will get bad results.
>>> Also the result will not be the least-squares best fit.
>
>>> Ciao,
>>> Paolo
>
>> It can still be fit as a linear system - just weight the residuals by
>> the measured values, like this:http://mathworld.wolfram.com/LeastSquaresFittingExponen tial.html
>
> Interesting... but I still do not see how they handle negative
> values...
>
> Ciao,
> Paolo

How about using yiprime = yi - min(yi) + eps ? Or does that change
the whole thing. It's too early - and I still have to get a cuppa.
I'll be back 8^]

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[Message index]

		Re: IDL - EXP fitting function By: glen_andy on Fri, 27 March 2009 09:13
		Re: IDL - EXP fitting function By: Vince Hradil on Fri, 27 March 2009 07:03
		Re: IDL - EXP fitting function By: pgrigis on Fri, 27 March 2009 06:50
		Re: IDL - EXP fitting function By: Vince Hradil on Fri, 27 March 2009 06:38
		Re: IDL - EXP fitting function By: pgrigis on Fri, 27 March 2009 06:27
		Re: IDL - EXP fitting function By: Maarten[1] on Fri, 27 March 2009 03:13
		Re: IDL - EXP fitting function By: Vince Hradil on Thu, 26 March 2009 19:11
		Re: IDL - EXP fitting function By: Christopher Thom on Thu, 26 March 2009 15:55
		Re: IDL - EXP fitting function By: glen_andy on Thu, 26 March 2009 15:33
		Re: IDL - EXP fitting function By: glen_andy on Thu, 26 March 2009 15:25
		Re: IDL - EXP fitting function By: David Fanning on Thu, 26 March 2009 15:15
		Re: IDL - EXP fitting function By: David Fanning on Thu, 26 March 2009 15:12

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