What does your data display the behavior of?

If I were studying Hooke's Law, I would plot force on one axis and
displacement on the other, and use the slope as a measure of the rigidity of
the material. The physical model for Hooke's Law is d = k F, where d is
displacement, F is force, and K is stiffness. The physical model for Ohm's
law is V = i R (i.e., voltage = current * resistance).

Knowing a little about the hypothetical basis for the relationship helps me
to decide what kind of curve fit to apply to these models. These are simple
linear models; most physical systems are more complicated, based on
exponential, power law, up to about second order polynomial, and similar
fits. When the fit requires more than second order polynomial fit, the
fitted model is almost guaranteed to be a "looks nice" model, but not
anything that is predictive of the underlying behavior. "Looks nice" fits
are okay to show something in conceptual terms, or if you need to
interpolate within the observed range of data. When you are studying the
underlying relationships, or if you are trying to extrapolate beyond the
observed data (always dangerous, especially with a 6th order poly fit), the
model had better have some physical basis for being selected.

- Jon
-------
Jon Peltier, Microsoft Excel MVP
Tutorials and Custom Solutions
Peltier Technical Services, Inc. - http://PeltierTech.com
_______



"jlm@mca" wrote in message
...
Sorry, had one other question. You mentioned needing to fit a physical
model
to my data. What does this mean?

"Jon Peltier" wrote:

1. Select the trendline equation, then use the Increase Decimal buttons
on
the formatting toolbar to add decimal digits, or press Ctrl+1 (numeral
one)
and click on the Number tab, and select a Scientific format with 15
places.

2. A 6th order poly fit is generally not well suited to fitting a curve
unless you are only looking to make the chart look pretty. As Martin
Brown
stated in another thread just an hour ago:

You really need to fit a physical model to your data rather than the
highest
order polynomial available.

- Jon
-------
Jon Peltier, Microsoft Excel MVP
Tutorials and Custom Solutions
Peltier Technical Services, Inc. - http://PeltierTech.com
_______


"jlm@mca" wrote in message
...
I used a 6th degree polynomial trendline to smooth a curve on a chart.
I
selected the option for printing the trendline formula on the chart so
I
could use it in my calculations, but the numbers in the equation are
rounded
so that I don't get the right results. Is there any way that I can
output
the equation with more significant figures shown?