Sanjay Kumar Limbikai wrote:

Sorry for confusing you,

Here it is more-
Excell 2002,
I was comparing coefficients with extracting using LINEST function to the
constants of displayed trend eq (6th order) with full precission. They are
all different.

Almost certainly the solution shown by the graphical chart polynomial is
a better fit than the one from LINEST. The simplest way to test this is
to compute the polynomial fit out for all your datapoints and then
explicitly sum up the squares of the residuals in a spreadsheet model.

Whichever one has more nearly minimised the sum of the squares is best.

Prior to Excel 2003 LINEST is potentially suspect on anything more
complex than a cubic polynomial for some nasty datasets. You might be
able to get better agreement between the two fits by using days since
1/1/1976 as the x variable (dates have a huge additive constant in
them). It should help improve the numerical stability of the problem a bit.

Regards,
Martin Brown