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Re: More problems with Curvefit [message #35711 is a reply to message #35619]

Mon, 30 June 2003 11:43

Craig Markwardt
Messages: 1869
Registered: November 1996

Senior Member

Paul van Delst <paul.vandelst@noaa.gov> writes:
> Try doing
> w = make_array(12, value = 1.0 )
> or
> w = replicate( 1.0, 12 )
> to actually give you a vector for w. The IDL documentation is a wee bit misleading here as
> the actual words say "For no weighting, set Weightsi = 1.0" where the "i" suffix is an
> indicator that Weights is an array but it's not entirely clear (to me at least.)

As a side note, MPCURVEFIT, and the other "MP" fitting programs can
accept either a scalar or a vector for the uncertainties or weights.
Either way it does the "right thing." And of course the MP fitting
programs do not necessarily require the user to compute function
derivatives.

To Heather Williams, the original poster, you should be aware that
your fitting function is degenerate, since it contains a linear
combination of the *same* basis function. This leads to a singular
normal matrix, and takes a canned routine like CURVEFIT on a trip to
la-la land. This effect is further exacerbated by the choice of
initial conditions (both coefficients equal). MPCURVEFIT is more
robust in this sense, since it will do a singular value decomposition
in the face of a singular matrix.

Happy fitting!
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]

		More problems with Curvefit By: heather.williams on Mon, 30 June 2003 07:35
		Re: More problems with Curvefit By: Heinz Stege on Tue, 01 July 2003 10:12
		Re: More problems with Curvefit By: heather.williams on Tue, 01 July 2003 04:54
		Re: More problems with Curvefit By: Craig Markwardt on Mon, 30 June 2003 11:43
		Re: More problems with Curvefit By: the_cacc on Mon, 30 June 2003 10:17
		Re: More problems with Curvefit By: Paul Van Delst[1] on Mon, 30 June 2003 09:19

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