Re: [Phys-l] error vs uncertainty

["David Marx" <marx@phy.ilstu.edu>]

Paul,

I would add that as we ask students to address sources of uncertaintty that we also want them to
estimate (if they do not calculate) the relative contribution to the total uncertainty that each source has.
What usually happens is that students will simply guess at sources of error without considering
whether these sources may be negligible or very substantial.  In my advanced lab class, I ask students
to not just  guess at uncertainties, but to back it up with either claculations and/or additional
measurements to see what the contribution to the variation of a measured value is from a given factor.

For example, in the e/m experiment we used to do, we had the electron beam curve around to strike a
series of 5 pins at different distances from the e-beam source.  Many students guess that the reason
they had a large percent difference between their value for e/m and the accepted value is due to their
inability to accurately determine where the pin was relative to the center of the e-beam.  I suggested
they go back to lab and make measurements with the beam on each side of the pins to see how much
their results varied due to this factor.  They were quite surprised to learn that this was not the source of
the error.

David


On 4 Oct 2011 at 11:15, Paul Lulai wrote:

> Hi.
> As someone has stated, students think of error as mistakes.  I emphasize uncertainty, and often pair that word with error.  They hear me say and ask about sources of uncertainty a lot more than they hear me ask about error.
> I ask my students to address sources of uncertainty for almost all of their labs.  I do not ask them to do calculations for all of them.  I do ask them to address what are some sources of uncertainty and how would they affect your data?
> At the beginning of the year I ask them how they could reduce the uncertainty of the lab they just finished.  After a couple of weeks I ask them to start thinking about that before they conduct the experiment.  Then they can include that in their discussion of uncertainties.
> We don't often calculate uncertainties, but they get a small taste.
>
> With freshman, we do use a method to find the uncertainty of a three beam balance, meterstick, and graduate cylinder.  They rotate through and find the mass, length, and volume of a set of standard objects.  We get a couple of classroom sets of data (close to 60 measurements).  Find the average, max and min.  We throw out any unrealistically large or small values.  We claim our uncertainty is the difference btn the average and the max (or min, whichever is larger).  Then we can discuss if things have definitely different, possibly the same mass, length, volume, or density values.
> I do state that there are other ways to determine uncertainty, and that this might not be the preferred method for folks everywhere.  We also discuss that electronic devices tend to have an uncertainty stated in their literature.
> It works well for high school freshman.
>
> I agree with an earlier statement. Human error isn't acceptable.  Also, students often want to state that uncertainty might be due to an error in calculations.  I tell them that is a mistake, not an uncertainty.  Fix the mistake.  While there may be some instances where these mistakes are hard to find, in high school, less than 5 minutes will typically find any mistake made in a calculation.
>
>
> Paul Lulai
> Physics Teacher
> St Anthony Village S.H.
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>
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>
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