Error in trendline equation for 3rd order and higher polynomials

[Cam Snow]

Dear Microsoft,

Excel 2007 has a bug in the trendline equations generated making 3rd order
and higher polynomials with the intercept = 0. When one makes a graph and
receives the equations y=-a*x^3+b*x^2+cx, the equation, when then entered
into cells to replicate the line does not yield the same answer.

This functionality is crucial for those of us that use excel as our graphing
package of choice. I was able to replicate the error on other data sets.
However, when using office 2003 it works properly.

Furthermore, if one clicks display equation, before changing from a 2nd to
3rd order polynomial, the equation maintains the form of a 2nd order
polynomial.

Thanks,

C

 

[Jerry W. Lewis]

As I recall, there are some issues with graph trendlines in 2007, but another
common problem is failure to display enough figures to have any chance of
calculating effectively with them. I'm not sure about 2007, but in previous
versions, you could right click on the equation and format to display in
scientific notation with 14 decimal places.

Also note that you can fit polynomials with LINEST. For example, to fit a
cubic with intercept forced to be 0, you would select 3 adjacent cells and
array enter the formula
=LINEST(y,x^{1,2,3},FALSE)

Jerry