Trendlines - 6th order polynomial

[Jonathan]

Does anyone know if there is a limit to Excel's capability
to accurate portray a 6th order polynomial trend line? I
have obtained a formulae for a series of noisy data and
got a nice 6th order polynomial curve with an equation. I
have then used this formulae and plugged in the same x
values and the graph apprears to exponentially increase
after some time and not come back down as the previous
graph showed.

Any help would be greatly appreciated.

Regards

Jonathan
Civil Engineer.

 

[Bernard Liengme]

The most likely explanation is precision. How many decimal places are you
showing in the Trendline equation? Remember that Ax^6 is going to be very
sensitive to A since x^6 is probably a large number. Right click on the
trendline equation; select Format and go to Number tap; have the equation
display 15 decimal places.
Better yet, don't bother with the copying step. Use LINEST to get the
coefficients into cells and generate the data using these. Select 7 cells in
a row and enter=LINEST(Y,X^{1,2,3,4,5,6}) /where Y and X are the ranges
for the data/ and complete the formula using shift+ctrl+enter.
More details on website
Best wishes
Bernard
www.stfx.ca/people/bliengme/ExcelTips

 

[Ron Rosenfeld]

Does anyone know if there is a limit to Excel's capability
to accurate portray a 6th order polynomial trend line? I
have obtained a formulae for a series of noisy data and
got a nice 6th order polynomial curve with an equation. I
have then used this formulae and plugged in the same x
values and the graph apprears to exponentially increase
after some time and not come back down as the previous
graph showed.

Any help would be greatly appreciated.

Regards

Jonathan
Civil Engineer.

If you are using the formula displayed in the equation, you are not displaying
enough precision. Format the Data Label to show the numbers with a higher
degree of precision (maximum is 15 decimals) and you should get a more accurate
formula.


--ron