Re: Increaseing Precision in polynomial trendline equations

Right click on the displayed trendline equation and format as scientific notation with 14 decimal places.

Often fitting a polynomial with this high a degree is overfitting the data. Even if the polynomial degree is theoretically justified, fitting it will often be an extremely difficult numerical problem, well beyond the capabilities of pre-2003 LINEST. If you provide your data (inline text, not attachments in newsgroups. please), I could provide more information.

Jerry

KevinW wrote:

How can I increase the precision in Excel's "Display Equation" option
for trendlines?

I am trying to fit a polynomial regression (trendline) to a X, Y
scatterplot I have created. 

Excel's built-in trendline function shows a good fit (R2=0.9999) for a
6th order polynomial, however, If I re-plot the data using the
coefficients from the "Display equation" option, the data diverge
significantly.

Apparently, for high order polynomials, you need to have a high level
of precision in your coefficients (many decimal places accurate) in
order to actually re-plot the same curve.

I also tried using the "linest" function as described in Mr. Liengme's
website (http://www.stfx.ca/people/bliengme/ExcelTips/Polynomial.htm),
however, I can't seem to make this work properly. I can make it
calculate, but the values don't match data. 

Any help with this problem would be appreciated.

Thanks
Kevin

PS Here is the equation that Excel displays on my chart;

y = 4E-09x6 - 9E-07x5 + 8E-05x4 - 0.0038x3 + 0.0996x2 - 1.5179x +
45.146

I am using Excel 2004 for Mac (OSX) Ver11.2




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