I am not familiar with the Blackboard program, but it seems like
it is calculating the standard deviation of the mean, rather than
the standard deviation of the distribution. The standard deviation
of the mean of N grades equals the standard deviation of the
distribution divided by the square root of (N - 1).
The standard deviation of the distribution is a measure of how
much individual grades vary from the mean. The standard deviation
of the mean is a measure of how much the mean of the same number of
grades is expected to vary if the same exam was graded for a
different group of students from the same parent population. In
this context, the same parent population means students with similar
academic backgrounds, perhaps in other schools or during different
school years.
Daniel Crowe
Oklahoma School of Science and Mathematics
Ardmore Regional Center
dcrowe@sotc.org
-----Original Message-----
From: Greg Clements [mailto:clements@MAIL.MLC.EDU]
Sent: Tuesday, November 11, 2003 11:07 AM
To: PHYS-L@lists.nau.edu
Subject: Re: Blackboard
Our school uses Blackboard 6. The standard deviation reported by Blackboard
is not correct. I checked one of my exams. Blackboard reports a standard
deviation of around 2. My statistics calculator computes a value near 13.
The mean reported by Blackboard is correct. n=24 for my class.
<snip>
Greg Clements
Midland Lutheran College
Fremont, NE