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First, we do not do percent error calculations, but % difference calculations. I believe Hugh's comments are more directly applicable to percent error calculations. I might be wrong.
My students are to determine why the two values might be different. I specifically state that we should not hold textbook values in too high of a regard.
They must state what might cause these differences, how their laboratory procedure might have caused these differences, and what could be done to change the procedure to limit the error involved. I do not give them the 'accepted' values of these substances.
Kids today will google or Wikipedia the value. If I don't ask them to do a % diff, some (not all) will attempt to fudge data to match the internet value. I've done labs along these lines to determine an unknown item in the past (maybe use n or C to determine what solution or metal sample you have been handed).
None of my labs are cookbook recipe labs. I rarely conduct verification labs. However, students are becoming very adept at finding some of the information that I ask them to determine. Years ago I would ask them to use a double slit to find the wavelength of a laser. Then kids could more easily find the wavelength. I switched to find the slit separation. Now kids google the little CAS image on the Cornell slides I've got and they know the openings on the slides. I now have to find a new version of Young's Double slit. Don't know what that will be, but it has never been a verification lab.
I never spoke of using human error as a source of uncertainty. I wouldn't accept it any more than you would.
I am not sure what Hugh means by an accepted value of 0.Any physical value that must be zero for certain situations or objects. For example, a collision in which the initial total momentum is zero. It must also be zero after the collision, but on an air track, it seldom is exactly zero due to factors that are very difficult to control. If you are asking them to calculate the % difference in before and after momentum, they will of necessity get an enormous value for that percentage. There are some physical properties of materials that have a theoretical value of exactly zero. Most of them are in advanced topics so first-year students are unlikely to encounter them but they exist. Measuring them involves a careful determination of the error bars of the experiment to determine if the theoretical value is included within the error bars. If so, there is no discrepancy with experiment and theory, so the experimenter will hope that they have narrowed the uncertainty in the experimental value. On the other hand, if the error bars do not include the theoretical value of zero, then it is important for the experimenter to be very sure that there is no remaining systematic error that can bring the value back to within the theoretical expectation, but if none is found, then something very important has been discovered--namely that there is something wrong with the theory. This is, of course, true for any theoretical value, not just ones that are presumed to be zero, but the zero values have a special nature in that one cannot identify the difference between experiment and theory with a percentage value.