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Re: CURVFIT with XY error [message #90200 is a reply to message #90199]

Tue, 10 February 2015 11:11

Craig Markwardt
Messages: 1869
Registered: November 1996

Senior Member

On Tuesday, February 10, 2015 at 1:33:18 PM UTC-5, rrya...@gmail.com wrote:
> Are you sure your function is nonlinear? I mean, is there any transformation you can apply to force it to be linear?

Things can get a little tricky when the function is non-monotonic. Linear, exponential and power law models are all monotonic, and will work fine with errors in X and Y, but you can also transform those into linear space and use MPFITEXY / FITEXY. This works because the function is single-valued whether it is expressed as y = f(x) or x = g(y).

Once your function has any bumps or wiggles, it's no longer a single-valued function in both x and y. Error bars in the x direction make it ambiguous whether a data point is attached to the left side of a wiggle, or the right side of it. This depends on how sharp the wiggles are, and how small the error bars are.

All this doesn't mean that fitting won't work, but it might mean that the fit converges to a local minimum which is not the global best fit. I.e., extra care is needed.

Craig

P.S. To Phil Brubaker, as already pointed out, the IDL subroutine is called CURVEFIT, and it has existed within IDL since 1982. Good luck with your trademark action.

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[Message index]

		CURVFIT with XY error By: amin farhang on Sun, 27 April 2014 14:31
		Re: CURVFIT with XY error By: Craig Markwardt on Tue, 29 April 2014 21:01
		Re: CURVFIT with XY error By: wlandsman on Mon, 09 February 2015 11:42
		Re: CURVFIT with XY error By: brubaker.phil on Mon, 09 February 2015 10:05
		Re: CURVFIT with XY error By: Craig Markwardt on Mon, 09 February 2015 10:10
		Re: CURVFIT with XY error By: brubaker.phil on Tue, 10 February 2015 08:42
		Re: CURVFIT with XY error By: dg86 on Tue, 10 February 2015 09:16
		Re: CURVFIT with XY error By: Russell[1] on Tue, 10 February 2015 10:33
		Re: CURVFIT with XY error By: Craig Markwardt on Tue, 10 February 2015 11:11
		Re: CURVFIT with XY error By: brubaker.phil on Fri, 13 February 2015 11:30

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