Re: How to get the equation of a curve fitting experimental data?


John D'Errico wrote:
> In article <ef21814.-1@xxxxxxxxxxxxxxxx>, Indira <iremond@xxxxxxxxxxx> wrote:
>
> > I am dealing with a set of experimental data y.
> > I would like to fit a curve to this set of data and get the equation
> > of this curve in order to reach the primitive function Y of y.
> > I tried 'polyfit' which actually gives the coefficients of the
> > fitting polynomial... the problem is:
> > 1- it doesn't fit that accurately the data
> > 2- I get the following warning:
> > Warning: Polynomial is badly conditioned. Remove repeated data points
> > or try centering and scaling as described in HELP POLYFIT.
> > > In polyfit at 79
> >
> > The cubic spline interpolation fits the data better... but it doesn't
> > provide any equation for the fitting curve.
> >
> > Is there any other built-in function that could help here?

--

> Typically one must start with some form of model.
> In essence, you must provide some sort of prior
> information.

And even then things are not necessarily easy.

I did some model-based studies of seismic waves. The idea
was to use a sum-of-sines model to investigate the spectrum.
Most model-based methods either need prior information
about how many waves were present (which was one
of the questions I wanted to answer), or need to estimate the
number of waves along the way. Which presents its own
problems: What about noise? Attenuation? Other model
imperfections?

Nah, model-based analysis is something of a black art.

> The best material model of a cat is another, or preferably the same, cat.
> A. Rosenblueth, Philosophy of Science, 1945

I second that!

Rune

.

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