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]

Error, a frank response!



--------------------------------------------------------
F. Lanzafame wrote (in part):
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.
(snip)
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.
-----------------------------------------------------------------
Mike Monce of Connecticut College responds (in part):

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.

Essentially, this is true. Sometime 0.2 % and sometime 5 %. The
achievable accuracy is a function of the method of analysis and
the equipment employed.

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.

Well, I cannot personally take credit for the "habits" with which
your students come into physics, but there is a certain symmetry.
I sometimes have to deal with "bad habits" my students have
learned in their physics classes. Tolerance of differences is
something we all deal with--even if it is a difference of
tolerances!

First, do you have the students evaluate why the established
technique is capable of yielding a 1% discrepancy?

Students in Analytical Chemistry begin with a fairly thorough
exposure to statistics, determinate and indeterminate error,
error propagation in calculation, etc. They begin their
laboratory work by calibrating their glassware. Since the
equipment is Class A and capable of 2 parts per thousand, it is
more an exercise in calibrating the student's use with accuracy
and precision than it is in actually calibrating the glassware.

If they get greater than a 1% discrepancy, are they then made
to determine the source(s) of the increased value?

I assume you are using 1% as an example. For chemical glassware,
1% is pretty crude; 0.2 % is more typically the range of
accuracy. The precision should be better than the accuracy.
During calibration of glassware, they get immediate feedback. If
they tighten their precision at that time AND maintain it, their
analyses are good. If they are careless, their results are poor,
but it is not always possible to determine the source of the
increased discrepancies. In later analyses their results may be
the result of very many measurements and observations over
perhaps 12 to 16 hours of laboratory work. It may arise from
determinate error, indeterminate error, or from some combination
of the two.

What are the limits of the supplier's value?

More than adequate. They have been in the business of supplying
student unknowns for quantitative analyses for many decades.

I still need to be convinced that such a grading technique is
not a pedagogical error.

With all due respect, I'm not certain that you need to be
convinced. It is only important that I am convinced.
Quantitative Chemical Analysis has a long and established history
of meticulous analysis for well over a century. I didn't begin
nor will it end with me.

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.

I am not "shocked" by your 9 +/- 1 example and I would expect the
statistics taught in a good Analytical Chemistry course should
prepare students to deal with error in measurement--determinate
and indeterminate. I am sorry that students arriving in your
physics classes do not fully meet your expectations. I cannot
help but expect that if they had learned the material in a solid
Analytical Chemistry text such as Skoog, West and Holler on
Errors in Chemical Analyses, Random Errors in Analyses, and
Application of Statistics to Data Treatment and Evaluation, they
would be well prepared.

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".

Students often approach each course with "tabula rasa", expecting
each instructor to begin from first principles with no
assumptions. Perhaps you could incorporate a bit more of Data
Treatment and Evaluation within your course.

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.

Your emphasis on the word "simply" leads me to suspect that there
is more to your question, but...???

But you should be aware of the ideas this plants in the
student's mind as to how experimental work proceeds.

I am aware that a good Analytical Chemistry Course with a solid
foundation in statistics, measurement and the critical evaluation
of experimental data is an excellent preparation for experimental
work. You seem to have a problem with someone or something.
Perhaps you should approach the source of your problems more
directly. I don't believe it can be found in the general nature
of a good Analytical Chemistry Course!


Ciao,


:-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-)
:-) :-)
:-) Frank M. Lanzafame Department of Chemistry :-)
:-) Monroe Community College 1000 East Henrietta Rd. :-)
:-) Rochester, NY 14623 (716) 292-2396 :-)
:-) Internet: flanzafame@monroecc.edu :-)
:-) :-)
:-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-):-)


I hope I haven't added to Mike's angst!