Re: what good is "percentage error"?

[PHRP@FHSUVM.FHSU.EDU (Roger A. Pruitt)]

Mike, you are being too hard on Frank. Don't kill the messenger. Take your
anger out on all of the chemistry departments around the world that grade
this way. Also, what are we to do about chemistry students in our classes
who want to "weigh" in mass units? It is an imperfect world we live in. We
do the best we can, and spend alot of time correcting misconceptions placed
in our students by others.

Roger

> On Mon, 12 May 1997, Lanzafame, Frank wrote:
>
> > In the quantitative analysis course, students are taught to analyze samples
> > with accuracy in the parts per thousand range.  Students often analyze a
> sample
> > whose composition is accurately known and supplied by a company specializing
> in
> > supplying such.
> >
> > IF a given sample is capable of being analyzed to an uncertainty of 2 parts
> per
> > thousand, then it is expected that the student perform at that level for
the A
> > grade.  The B, C, etc. grades are assigned to increasing deviations from the
> > "true" or accepted value for that sample.
> >
> > I'm not certain of the origin of your "horror", but the grade is designed to
> > reflect the accuracy with which the analysis is performed in a course
which is
> > designed to teach quantitative methods.
>
>
> 	So this grade is based purely on evaluating their technique only
> with the assumption that if their technique is proper they will achieve a
> result to within 1% of the supplier's value..  I have several questions
> about this, based on my experience in spending a year trying to undo some
> of the habits these students come into physics with.  First, do you have
> the students evaluate why the established technique is capable of yielding
> a 1% discrepancy?  If they get greater than a 1% discrepancy, are they
> then made to determine the source(s) of the increased value?  What are the
> limits of the supplier's value?
>
> 	I still need to be convinced that such a grading technique is not
> a pedagogical error.  The students arrive in my physics classes convinced
> in the "rightness" of any value given in the book.  They are obsessed with
> the "percent error".  They are shocked to learn that if they measure a
> value for g to be 9 +- 1 m/s^2 that their value is actually in agreement
> with the "accepted" value, as long as their uncertainty is an accurate
> calculation of the limits of their measuring system.  They never seem,
> especially at the beginning, to be able to question the accuracy of their
> equipment, or the experimental technique being used.  All "percent error"
> is attributable to "human error".
>
> 	Is the purpose of the course simply to teach established
> techniques and methods which are regarded as simply skills a chemist
> needs?  Then, maybe, the grading is appropriate.  But you should be aware
> of the ideas this plants in the student's mind as to how experimental work
> proceeds.
>
>
> 					Mike Monce
> 					Connecticut College
==============================================================
Roger A. Pruitt, PhD
Professor of Physics
Fort Hays State University
Hays, KS  67601
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