Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: what good is "percentage error"?




Dear Pete:
You write:
***********************************************************
My students are allowed to use ERROR only when there is a known value to
compare their lab results to. If they attempt to determine the
acceleration due to gravity in the lab, they can report that they found
it to be 9.71 m/s/s an absolute error of 0.10 m/s/s or that they
measured the value to within 1% of the accepted value, or that their
value only had a 1% error.

If there is not an accepted value to compare to, then an uncertanty is
expected on the report.

Do I need to change my thinking here?
**********************************
Yes. I think so.
There are several separate issues involved here:
1. In a typical beginning lab experiment, if the students handle
significant digits correctly, they will all get very nearly the same
answer. One source of fluctuation from "accepted values" is the
incorrect use of significant figures. I assume that you have
taken this into account.
2. The value of "g" varies from point to point over the surface of
the earth. Centrifugal acceleration (I'm sure that you know what I
mean) contributes at the level of .01 m/s/s, depending upon your
latitude, for example. The beginning student need only know, however,
that the local value of g is unknown. It is therefore incorrect to
give the student an "accepted value" to two places for comparison.
3. Large deviations from 9.7 signal that the experiment, or the
calculations were done incorrectly. What one might do, and I have
done, is to give the student a range of values obtained by other students
who did the experiment correctly, like 10.68 pm .08. You might then
keep all of the values of all of the results that were not obviously
incorrect to obtain a new range. The student who gets a large deviation
from the given range should reexamine the experiment to find where
s/he went wrong.
regards,
Jack

"I scored the next great triumph for science myself,
to wit, how the milk gets into the cow. Both of us
had marveled over that mystery a long time. We had
followed the cows around for years - that is, in the
daytime - but had never caught them drinking fluid of
that color."
Mark Twain, Extract from Eve's
Autobiography