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Re: what good is "percentage error"?



Time for my soapbox:

On Mon, 12 May 1997, John E. Gastineau wrote:


Today I was looking through a commercial high school physics level lab
manual, and came across the instruction to perform an experiment once
and then determine the "percentage error" by calculating

(accepted - measured)/accepted * 100%.

I have no idea what this means either. "Error" to my mind means
that something is wrong, and I have never been able to figure out
why students should try to do an experiment as carefully as possible and
then try to figure out how "wrong" it is by calculating "the error".

I have deliberately avoided using "error", and have always asked
students to calculate or estimate the "Uncertainty" in their result.
Nothing can be measured exactly, and thus an experimental result should
be defined by a range of values. If the "accepted" value lies in this
range, then there is no error. Of course, a small range of uncertainty
means that the individual uncertainties are small, and the result
is more accurate than one with a large range of uncertainty.

If the "accepted values" lies outside this range, then there is an error,
and it should be up to the student to figure out whether it is a real
effect, or due to a calculation mistake.


I think students use it because they have it memorized, and think they
understand it. Getting them to unlearn it has been very rarely successful.

Why? What good is it? Am I missing something?
Me too. In particular, I want to know what this formula means in the
context of a laboratory experiment, and what physical meaning should
be assigned to the result??

Chris

+-----------------------------+-------------------------------------+
| Christopher Deacon | (709) 737-7631 |
| Dept of Physics and Physical| cdeacon@kelvin.physics.mun.ca !
| Oceanography | |
| Memorial University of Nfld | http://www.physics.mun.ca/~cdeacon |
+-----------------------------+-------------------------------------+