Excel trendline

[william ballard]

On May 15, 2006, at 1:11 AM, John Fortiner wrote:
> Ron:  The "smoothing" is simply a "rounding off" of values.   
> Sometimes it
> rounds up and sometimes down depending on settings.  Sort of like  
> saying the
> over tension is "about" 40k lbs on a piano.  That is a "smoothed"  
> figure.
> It is totally impossible to derive an "exact" from that.  To derive  
> the
> "exact" a person would have to have all the raw data to recalculate
> again.....

Of course the raw data is available one or two columns over. In the  
program that I've always used for charting and statistical smoothing  
(polynomial or otherwise), DeltaGraphPro ( http://www.rockware.com/ 
catalog/pages/deltagraphw.html ), the specific values are available  
by copying the formula to another column, and then making a few quick  
manual edits.

The process is the same as described by Ron. You tell the program  
here is the column whose data was smoothed, and here is the formula  
(including such specifications such as the polynomial degree, the  
number of bezier nodes used to draw the curve, and whether or not  
point on the x-axis, say #0,#89 and#90, which have y-axis value of 0,  
should be allowed to skew the curve.) You then ask the program to  
return the y-values of each x-axis increment as it intersects the  
curve-fit.

Of course, my math skills take an irreversible hop from the left  
brain to the right, somewhere soon after the basic concepts of the  
integral and differential (....never did figure out what makes "e"  
such a useful benchmark in higher mathematics, even though I once  
read an entire popular math book on it .....).

Mr. Bill