Ladies and gentlemen,
In a course I just taught, the lowest of four test grades
was to be dropped. This is done in my grading worksheet
by forming the expression
=SUM(L2:O2)-MIN(L2:O2)
where the test data is in L2, M2, N2, and 02.
Most students took all four tests, but some took only three tests.
I created the spreadsheet with Excel, and if a student had not
taken a particular test I left the corresponding cell blank.
Then I transferred the grading ".xls" spreadsheet to a Linux
system (I much prefer Linux) and started working on the
spreadsheet with oocalc (OpenOffice.org 2.0).
Now for the GOTCHA: The grades for the students who took only three
tests, suddenly decreased! If I hadn't noticed what happened,
I could easily have given students incorrect grades.
The explanation is as follows:
MIN under Excel considers blank cells to contain zero data, but
MIN under oocalc ignores blank cells.
Suppose the test grades were 80, 60, (didn't take), and 70.
With Excel, the MIN of the four test grades is 0
and the sum of the three best test grades is 210.
With oocalc, the MIN of the four test grades ignores the blank
cell and finds the minimum of 80, 60 and 70, that is, the 60.
The sum of the three best test grades is wrongly computed as
(80+60+70)-60 = 150.
Folks will respond: "Why not just put zeros in the spreadsheet for the
missing data?"
Well, it's a question of taste. I prefer to use blanks for cases in
which a student missed a test, to reserve the number 0 for the case in a
student took a test and got 0 points.
IMHO, in this case Excel does it right and oocalc does it wrong.
I have made several attempts to get people to convert from Windows to
Linux, but issues like this one in which just moving a grading
spreadsheet might cause students to end up with incorrect grades, are
real deal breakers.
Brian Hassard,
Department of Mathematics,
University at Buffalo, SUNY
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