Treatment of Numerical Data - Exp 2 (dTf)

[robert zellmer]

I always get a number of questions about Exp 2 and what to do for the
average MW if two of the trials are close and one isn't.  Should you include
all three in the average in this case?

Sometimes the answer is pretty obvious.  If you have 150, 155, 220 hopefully
you realize something may be wrong with the 220 since the other two are
so close.  I suppose it could happen the 220 is the correct one and somehow
one got two wrong results out of three which were close but this would be
unusual (at least we hope).  In this case the 220 would not be included in
calculating the average MW.  In the discussion section one would list 
all three
MW and the average and explain the one MW was left out of the calculation
of the average.

Sometimes what to do isn't quite so obvious.  What if the results were 150,
160 and 190.  Should the 190 be included.  If you do the average rounds to
170 (2 s.f.).  If you don't it rounds to 160 (2 s.f.).  Which is 
correct?  I would
probably go with the 160.  However, to get a correct answer one needs to
do an error analysis and see if the 190 should be included in getting the
average.

I have a link under the "Laboratory" link.  This is a copy of Appendix F
in the current lab manual.  Here's the direct link,

*Treatment of Numerical Data (Error Analysis, sig. fig., graphing)* 
<http://chemistry.osu.edu/%7Erzellmer/chem1220/lab/App_F_1220_lab_manual.pdf>

The discussion about how to determine if a data point can be ignored in
such cases is discussed in Section III, "Reporting Results" on pages F-3
through F-5.

By the way, I can't tell you if leaving in one "bad" experimental result 
will lead
to a better or worse average.  That depends on your results and how careful
you were being.  It is quite possible for the two "good" results (they 
are very
close to each other) to actually be incorrect.  Generally if you get two 
results
which are very close to each other and one which isn't one would expect the
two which are close to be better.  However, this isn't always the case.  
It is
quite possible you made a mistake and made that same mistake twice so the
two "good" ones aren't "good".  Besides, I don't know what any of the actual
true results are for any of the experiments.  Even if I did, I couldn't 
tell you
under penalty of death (well maybe not that severe a punishment but close).

For the post-lab data entry you have to enter your data as is (i.e. all 
three
MW values) and the average for all three (including the "bad" one). Then
in the data table in the report template, for the average list what you 
get from
just the two "good" MW values.  Discuss this in the Discussion section and
leave a comment when you submit it asking the TA to grade the average MW
based on your report sheet, if you desire.

Dr. Zellmer
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