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Topic: Which interpolation?
Replies: 4   Last Post: Dec 10, 2012 12:22 PM

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 Gordon Sande Posts: 133 Registered: 5/13/10
Re: Which interpolation?
Posted: Dec 7, 2012 3:14 PM

On 2012-12-07 14:13:12 -0400, Cristiano said:

> From a very slow simulation I got y= f(x):
> x y
> 5 0.8048174
> 6 0.8194384
> ...
> 44 0.9706268
> 47 0.9724846
> ...
> 48765 0.9999756
> 53765 0.9999776
>
> For every x, I stop the simulation when the confidence interval for y
> is less than 2,5*10^-6 (with 99% of confidence).
>
> I can't calculate all the x's (because the simulation is very slow), so
> I need to interpolate; for example, I don't have y(45) or y(46).
>
> Using the Levenberg-Marquardt Least Squares Fitting, the best equation
> I found gives an error that is too high (about 10^-4 for small x's).
>
> Then I thought to use a cubic spline, but I notice some "fluctuations"
> on the tails.
>
> Should I use LM or spline?
>
> Thanks
> Cristiano

For this type of problem it helps greatly if you have an approximate
notion of what
the function looks like. Then you fit either a multiplicative or
to the approximate functional form.

You did not say what you were fitting to the data but most classes of
functional
fits get their results by oscillating between the observed values. That is what
they are intended to do if you read enough explanations.

By the way, there are special variants on splines that are monotone. Assuming
that you know that yu are fitting to a montone result. See first point!

Date Subject Author
12/7/12 Cristiano
12/7/12 Gordon Sande
12/7/12 Dave Dodson
12/8/12 Cristiano
12/10/12 Peter Spellucci