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 fit a parabolic curve (data provided)		  New Topic  Reply to Topic
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right

Finland
2 Posts
Posted - 06/16/2003 :  02:42:08 AM  Show Profile  Edit Topic  Reply with Quote  View user's IP address  Delete Topic
I need to find the equation of a dataset that i have, when plotted looks like a parabolic curve.

here is the data

x y
0.1 28.8707
0.2 17.3049
0.3 13.4001
0.4 11.4265
0.5 10.234
0.6 9.42635
0.7 8.86566
0.8 8.43657
0.9 8.11447
1 7.85629


I've tried the non-linear curve fitting by creating my own equation P1/(p2+x), but i can't seems to get a nice fit for my curve.

Can someone please help or is there is other ways to do this?


thanks

easwar

USA
1965 Posts
Posted - 06/16/2003 :  09:44:12 AM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
Hi,

If you wish to use NLSF, there is a polynomial category that has a built-in parabola function that you can make use of. The data does nt look very "parabolic" - perhaps you can get better/desired fits if you fix parameters and/or place lower/upper bounds etc. if doing so makes sense for your data/measurement.

Note that you can also use the Analysis->Fit Polynomial menu to perform the fit.

Easwar
OriginLab.
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right

Finland
2 Posts
Posted - 06/16/2003 :  10:20:30 AM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
but I am expecting a parabolic curve.
can someone please guide me on how to do that.
I know how to use a polynomial fit, but that's not what i want.
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Mike Buess

USA
3037 Posts
Posted - 06/16/2003 :  10:42:25 AM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
P1/(P2+x) works fine in Origin 7 if I uncheck Use Origin C and use Expression for the function form. Even so, your data fit an exponential much better.

Mike Buess
Origin WebRing Member

Edited by - Mike Buess on 06/16/2003 10:43:11 AM
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edgar.kaiser

Switzerland
29 Posts
Posted - 06/18/2003 :  04:48:54 AM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
Hello Right,

your data looks like a hyperbola and not a parabola. Also your own model function is hyperbolic with a pole at x = -p2.
A parabola has a model function y = p0 + p1*x + p2*x2

Maybe you should introduce an y-offset into your hyperbolic model:

y = p0 + p1/(p2 +x)

This model is not forced to approach zero for large x-values. Also an exponential with y-offset might work as Mike suggested.

Regards,

Edgar
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