First off, thanks to Bernard, Tushar & Jerry for their helpful comments

and suggestions. I learned alot about a useful function (LINEST) and

regression curve fitting in general, something I hadn't expected when I

posted my question.

Right click on the displayed trendline equation and format as

scientific

notation with 14 decimal places.

It worked, and that was exactly what I had been trying to do

originally. With more (numeric)precision, my re-ploted data matched my

original line exactly.

I find that sometimes Excel is 'fussy' about whether a text box is

already selected when you open a formatting menu - sometimes it only

shows the "Font" menu and not the "Colors &

Lines/Font/Number/Alignment" menu depending on exactly what you have

selected. Once I figured that out, I was able to increase the displayed

precisions as I needed.

One general observation (which might be old news to some)- whenever I

work with trendlines and curve fitting, I find that Excel does not

always properly 'refresh' the trendline equation on the chart. If I

switch curves (exponential to log etc) or change the order of the

polynomial, the equation doesn't change, or shows something different

than If I plot an entirely new trendline with the same fitting

equation.

This was the case when I tried your (Jerry's) suggestion as well. I

increased the precision on the equation I had showing in Excel, and

plotted it, but the data diverged again. When I set a new trendline and

compared the equations, the coefficient of the 4th term was completely

different, and the new equation worked properly.

I'd still like to be able to use the LINEST function sometimes in the

future, but until I can figure out my issue with replicating Bernard's

example,

(<

http://www.stfx.ca/people/bliengme/ExcelTips/Polynomial.htm>)

I'll have to be cautious. I will be back at my office this week, and

will try the example on my Windows computer and report back on any

results. For simplicity's sake, I hope the problem was between the

keyboard and the chair.

Thanks again

Kevin

Often fitting a polynomial with this high a degree is overfitting the

data. Even if the polynomial degree is theoretically justified,

fitting

it will often be an extremely difficult numerical problem, well beyond

the capabilities of pre-2003 LINEST. If you provide your data (inline

text, not attachments in newsgroups. please), I could provide more

information.

Jerry

Just to be clear, I am -not- trying to find the 'best fit' for a plot

of scattered data. Rather, I am trying to find an equation to describe

an existing line. I scanned and digitized a Larson-Miller curve I will

be using extensively for my thesis. The data points are very close

together. I'd like to be able to enter a value and return the

corresponding value from the curve. As long as the answer is the same

as the original curve, then I'm happy with the equation for the line.

Digitizing and inputing the curve into Excel is probably more accurate

than trying to manually read values over and over from a hardcopy

plot.

Here is an example of my data;

Row X Y

1) 30.07 62.08

2) 30.08 62.08

3) 30.09 62.08

4) 30.09 61.96

5) 30.10 61.96

6) 30.11 61.83

7) 30.12 61.83

8) 30.13 61.71

...(snip 1000 data points)....

1047) 38.81 6.25

1048) 38.82 6.25

1049) 38.83 6.24

I may want to know what the 'X' is for Y=61. Since my digitizer didn't

input a number for exactly 61, I would have to interpolate, maybe using

some sort of look up table etc or just use something close. Either way

it would be slow and semi-manual. However, Since I have *alot* of data,

the polynomial equation equation fits the line well (at least within the

accuracy of the scan etc). I can get my X for any Y I select (but bound

by {30.07,62.08} and {38.83,6.24} i.e. no extrapolating).

I hope that makes sense - if you like I could still post the data,

however there is alot of it.