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Re: what good is "percentage error"?



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It appears that we are talking apples and oranges here. My point has
been that we cannot replace proper accuracy estimates (a more positive
term than "error analysis") with a discrepancy between the obtained value
and an accepted value. If you grade according to "percent error" as
compared to an accepted value, and the students know the value, you will
find everything they do is driven towards getting that value so that the
grading becomes a farce. I consider it perfectly legitimate to use
"unknowns" (which the marker knows but the student doesn't) as a test of
the true experimental skills of the student. However, when doing so the
marker should also have some proper analysis skills. Suppose that you
somehow did a measurement of 'g' without any of the students knowing the
"right" answer is 9.8. Your own experience with the equipment says that
the best you can get out of it is a value of +/- 0.1. Then students who
get 9.7, 9.8, and 9.9 should all receive the same mark for accuracy.
Markers very often are unaware of this distinction.
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You're right we were talking apples and oranges. I basically agree with the
above. Although, I do think the student should try to get the "right"
answer even when known in advance; WITHIN THE LIMITS OF THE EQUIPMENT. And
I'm experienced enough to know that under these circumstances a lot of
students will get that answer by hook or crook. This is of course where you
need an alert lab instructor and evaluator who can distinguish between
legitimate methods and "fudge" methods of obtaining the "right" answer.

I fear that little emphasis on accuracy, which tends to happen with "we
won't grade your results" style grading tends to tell the students that
sloppiness doesn't matter, because after all your grade won't suffer when
you obtain 9.4 m/s^2 for your measurement of g as long as your error (errrr,
discrepancy analysis) defends it. (I assume in the above extreme
statement that the equipment and setup and procedure really does allow a
careful measurement to say within P/M 0.1

Obviously, if you can only get P/M 1.0 with the equipment then 9.4 is a
perfectly fine answer.

Joel