Results of trend line in Excel

[Guest]

I fitted some data using the trend line tool. The fit purportedly was a 3-rd
order polynmial. When I create a column of values using the coefficients
that Excel found (Display equation), I find the data does not overlay the
trend line. I created data in the range of the fit so there is no error due
to extrapolation and the fit R value was 0.9999. Is there an error in the
routine that reports the coefficients for polynomial fits? If so, is the re
a patch or work around?

Thanks

 

[Bernard Liengme]

The most likely cause is that you are working with two few digits. If you
format the trendline equation to display more digits I am sure you will find
all is well,
Of course, this problem of copying values from the equation is fraught with
problems - typos resulting from arthritic fingers or from poor reading
glasses being some of them
Best to use LINEST to get the coefficients into cells; see
http://www.stfx.ca/people/bliengme/ExcelTips/Polynomial.htm

best wishes