Polynomial is badly conditioned

Saga

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Mar 4, 2010, 9:12:05 AM3/4/10

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Guys trying to create a polynomial formula but I go above order 5 i get this error msg "Warning: Polynomial is badly conditioned. Add points with distinct X
values, reduce the degree of the polynomial, or try centering
and scaling as described in HELP POLYFIT."

I've looked at help, but there is no information there.

Thanks

Steven Lord

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Mar 4, 2010, 9:36:12 AM3/4/10

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"Saga " <a5stargam...@googlemail.com> wrote in message
news:hmof3l$851$1...@fred.mathworks.com...

Reread this section from the help:

[P,S,MU] = POLYFIT(X,Y,N) finds the coefficients of a polynomial in
XHAT = (X-MU(1))/MU(2) where MU(1) = MEAN(X) and MU(2) = STD(X). This
centering and scaling transformation improves the numerical properties
of both the polynomial and the fitting algorithm.

--
Steve Lord
sl...@mathworks.com
comp.soft-sys.matlab (CSSM) FAQ: http://matlabwiki.mathworks.com/MATLAB_FAQ

Saga

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Mar 4, 2010, 9:48:03 AM3/4/10

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"Steven Lord" <sl...@mathworks.com> wrote in message <hmoggi$as7$1...@fred.mathworks.com>...

OK I understand it, but have no idea how to apply the method?

Thanks

Steven Lord

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Mar 4, 2010, 10:15:10 AM3/4/10

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"Saga " <a5stargam...@googlemail.com> wrote in message

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Call the function with three outputs.

Saga

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Mar 4, 2010, 5:24:22 PM3/4/10

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"Steven Lord" <sl...@mathworks.com> wrote in message <hmoipk$av0$1...@fred.mathworks.com>...

Three outputs???

I only have (x,y,n)

The higher order I go it display the error.
Do you mean like this?

[P,S,MU] = POLYFIT(X,Y,N)

If yes what values are P,S and MU?

Thanks

pipa

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Mar 4, 2010, 6:42:23 PM3/4/10

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> Three outputs???
>
> I only have (x,y,n)
>
> The higher order I go it display the error.
> Do you mean like this?
>
> [P,S,MU] = POLYFIT(X,Y,N)
>
> If yes what values are P,S and MU?

P, S and MU are the three outputs which u will get after you use the polyfit function.

Saga

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Mar 5, 2010, 6:03:02 AM3/5/10

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"pipa " <balwind...@gmail.com> wrote in message <hmpggv$oqg$1...@fred.mathworks.com>...

Thanks but how would I use these values?

Steven Lord

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Mar 5, 2010, 9:38:47 AM3/5/10

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"Saga " <a5stargam...@googlemail.com> wrote in message

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That depends on what you want to do with the polynomial after performing the
fitting -- me, I'd probably use then in a call to POLYVAL.

Saga

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Mar 7, 2010, 4:05:24 AM3/7/10

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"Steven Lord" <sl...@mathworks.com> wrote in message <hmr51c$rqm$1...@fred.mathworks.com>...

When I don't use P, S and MU I write a = polyfit(x,y,6)

then to calculate the polynomial I write polyval(a,x)

Where would I insert P,S and MU?

Thanks

Steven Lord

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Mar 7, 2010, 11:15:49 PM3/7/10

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"Saga " <a5stargam...@googlemail.com> wrote in message

news:hmvq8k$9i0$1...@fred.mathworks.com...

> "Steven Lord" <sl...@mathworks.com> wrote in message
> <hmr51c$rqm$1...@fred.mathworks.com>...

*snip*

> When I don't use P, S and MU I write a = polyfit(x,y,6)
>
> then to calculate the polynomial I write polyval(a,x)
>
> Where would I insert P,S and MU?

Read the help for POLYVAL -- it describes specifically how you use P, S, and
MU in a call to POLYVAL.

chiara dec

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Jun 27, 2017, 7:58:08 AM6/27/17

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>
> Where would I insert P,S and MU?
>
> Thanks

You can find an example of the implementation at the end of this page:
https://ch.mathworks.com/help/matlab/ref/polyfit.html

hope it helps!