Excel graphing and equation issues

[danklebs]

Okay so here is my problem. For work, I have a curve that I need to put into
excel and find the equation. So I look at the curve and record data points
from the curve into excel. I will then graph the points and it will produce a
curve just like the one have on paper. My problem comes when I ask excel to
produce equations for the curves. The equations it produces do not match the
curves when I enter a value into the equation. Why does this happen or how
can I fix it?

 

[Jon Peltier]

Make sure you start with an XY chart, not a line chart. Then make sure
the equation has enough digits, so apply a scientific number format with
15 decimal places.

- Jon

 

[PBezucha]

Better let’s repeat your procedure. You read out several coordinate pairs on
a curve from some graph. You transferred them into an Excel spreadsheet. You
created a xy-chart. In it, you created a regression line (“trend lineâ€), for
example a quadratic one: y = a0 +a1*x + a2*x2. You read the regression
coefficients a0, a1,… from the text box pertinent to the series of your
points. You tried to write the formula in Excel having the same form as the
regression function. You found out that the results after substituting x’s do
not give the expected results. Most probably you are making some basic
mistake (wrong constitution of the equation,+/-, wrong transferred
coefficients etc.). First after eliminating such trivialities you can test
the influence of insufficient accuracy. More transparent method is a direct
statistical treatment of data (LinRegression and similar functions).

 

[Bernard Liengme]

Rather than copying the trendline parameters, use LINEST to place their
values in worksheet cells. This gives you full precision and the values get
updated should you correct the charting data.
For more see
http://people.stfx.ca/bliengme/ExcelTips/Polynomial.htm
http://people.stfx.ca/bliengme/ExcelTips/ExponentialTrendline& LOGEST.htm
best wishes