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Re: IDL - EXP fitting function [message #65901 is a reply to message #65898]

Fri, 27 March 2009 03:13

Maarten[1]
Messages: 176
Registered: November 2005

Senior Member

On Mar 26, 11:55 pm, Christopher Thom <ct...@oddjob.uchicago.edu>
wrote:

>> 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]

Be very careful with that. Small offsets (on measured data, usually
way below the noise level) will throw this fit way off. A non-linear
fit will probably give better results, especially if you have a
realistic error model (Poisson noise on a decay signal for instance).

At the very least plot the fit over the data, and plot the residuals
for visual inspection.

Maarten

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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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