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Re: what good is "percentage error"?
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From: phys-l@atlantis.cc.uwf.edu at internet@CCMNRC
Date: 05-13-97 12:23:00 PM
Subject: Re: what good is "percentage error"?
While we've beaten the 'error' part to death (without, I think, a clear
winner), let me go back to the percentage part.
An example of why getting students to think in terms of percentage
differences (uncertainties, errors) is important can be illustrated using a
simple experiment. In the first lab of the year, I have students
determining densities and hence the material of several objects. One of
those objects is a long (50-60 cm) copper wire with a diameter in the 1-2
mm range. The wires have been bent for storage. The students use a dial
caliper to get the diameter and a meter stick for the length and generally
don't get a very good match to copper for the density even though (from the
color) many do suspect that the wire is copper. When asked to comment on
their findings, most will blame the length measurement for the problems
they had with the wire--it was hard to straighten and they had to use a
meter stick--while the diameter measurement seems quite accurate to them
(after all, the dial caliper can be read to .01 cm!) The teaching moment
then presents itself to have them calculate the percent uncertainty in the
length measurement even if it is only good to +/- 1 cm, versus the
uncertainty in the diameter if good to +/- 0.1 cm. The absolute
uncertainty in the diameter may be 1/10 that of the length but the
percentage uncertainty is 5 times greater.