interpolation from a curve in excel 1

[mechmaj]

dear friends,
I have a curve, but i have not any formula or function,
how can i find a good fitting function and extract new data from this curve.
for example:
from a curve we can find that:
f(a1)=b1
f(a2)=b2
f(a3)=b3,...
then f(x)=?
Thank you,

 

[jartgo]

If the curve has been plotted by excel, then you can fit a "best line" through it (using Excels tools) and just use the equation of the best fit line to find your value of interest. If the curve is on paper, then you need to first create the curve in excel just like you did in this post.

 

[TerryScan]

I found the built in functions lacking for certain purposes. After research and looking at a few add-ons, I found this one to be great:

http://preview.tinyurl.com/2prpo4

From the website:
"Three Dimensional Interpolate and Extrapolate Functions; Two Dimensional Interpolate, Extrapolate, Cubic Spline, Polynomial Curve Fitting, Line Intersections, Minima, Maxima, First Derivative, Second Derivative, and other functions."

 

[IRstuff]

What's the model? The data must represent something with a physical process that has some model. You curve fit to the model.

TTFN

FAQ731-376

 

[AlmostPE]

do trendline equation?

Not almost anymore!

 

[corus]

I've fitted a curve of best fit to data in the past, or if it's difficult, derived my own least squares fit (say for a cosine function or a circle) and then cut and pasted the formula into cells to calculate any point on the curve. You have to edit the pasted term though to make sure it refers to a cell that contains the 'x' value.

corus

 

[stick1]

Mechmaj,

You can also turn to other sources beside Excel for curve-fitting. For some online tools, check out:

http://www.zunzun.com/

http://www.savetman.com/curvefit/index.html

http://www.softintegration.com/chhtml/lang/lib/libch/numeric/CGI_Curvefit.html

http://www.xuru.org/rt/TOC.asp

I have used zunzun.com to perform the same task. As stated previously, it helps to know the model that the data came from.

 

[zunzun]

Thanks for mentioning my site, zunzun.com. The source code for the middleware - the code that iteratively searches for the 'best' equation - is at

http://sourceforge.net/project/showfiles.php?group_id=187105

If you would like to make your own curve and surface fitting web site, the tutorial for this is at

http://wiki.pylonshq.com/display/pylonscookbook/Pylons+Curve+and+Surface+Fitting+Tutorial

Please let me know if you have any questions or suggestions for the site.

James