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Re: Newbie's question [message #45901 is a reply to message #45898]

Thu, 20 October 2005 15:08

James Kuyper
Messages: 425
Registered: March 2000

Senior Member

ChiChiRuiz@gmail.com wrote:
> Poly_fit doesn't really give me what I need. I don't need the
> coefficients of a quadratic equation, I want to know the best fit of
> the scatter plot to some power of x. I know it's not exactly power
> square, but it should be in that neighborhood. Even if I shift all
> data to the positive axis, i.e. y = a* (x-x0)^b, any x values less than
> x0 is still considered "negative". I don't know what else...maybe I'll
> try change of variable or something... thank you for your help.

What leads you to believe that y is some power of x? Is it simply a
guess based upon the shape of the curve, or do you have some
theoretical reason for expecting a power relationship?

Theories that lead to a power-law relationship without fixing the power
to be a specific rational number generally apply only to data where the
dependent variable is guaranteed to be positive. The numerical problems
you have trying to fit such a relationship to data where x is sometimes
negative are directly related to the reasons why theories tend not to
imply the existence of such relationships.

For that same reason, if you're merely guessing at what the shape of
the curve is, rather than getting it from a theory, I suspect that your
guess is a bad one.

One possibility: the relationship isn't y = a*x^b; it's actually y =
a*|x|^b. I've seen situations where that is a reasonable model. That
will avoid the problems you've been having.

This is really a scientific problem, not a numerical one; figure out
the right model for your data and the curve-fitting routines shouldn't
have any problem fitting it to your data.

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[Message index]

		Re: Newbie's question By: JD Smith on Thu, 20 October 2005 17:07
		Re: Newbie's question By: James Kuyper on Thu, 20 October 2005 15:08
		Re: Newbie's question By: ChiChiRuiz@gmail.com on Thu, 20 October 2005 13:18
		Re: Newbie's question By: James Kuyper on Thu, 20 October 2005 11:41
		Re: Newbie's question By: Paolo Grigis on Thu, 20 October 2005 08:10
		Re: Newbie's question By: Paul Van Delst[1] on Thu, 20 October 2005 07:10
		Re: Newbie's question By: ChiChiRuiz@gmail.com on Thu, 20 October 2005 06:53
		Re: Newbie's question By: Paul Van Delst[1] on Thu, 20 October 2005 06:44
		Re: Newbie's question By: peter.albert@gmx.de on Thu, 20 October 2005 04:02
		Re: Newbie's question By: Paolo Grigis on Thu, 20 October 2005 00:33
		Re: Newbie's question By: ChiChiRuiz@gmail.com on Fri, 21 October 2005 09:58
		Re: Newbie's question By: James Kuyper on Fri, 21 October 2005 05:55

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