Re: Blackboard

[Ludwik Kowalski <kowalskil@MAIL.MONTCLAIR.EDU>]

On Tuesday, Nov 11, 2003, Folkerts, Timothy wrote:

> Within any specific knowledge of what Blackboard is doing, I might
> venture
> a guess that Blackboard is reporting what is known as the standard
> error,
> which is (st_dev)/(n^1/2).
>
> This seems to basically agree with your numbers:  13 / 24^(1/2) = 2.6
> > Blackboard reports a standard deviation of around 2.
> > My statistics calculator computes a value near 13.
>
> The standard deviation is a measure of how similar the students within
> a
> section are.  The standard error is an estimate of how similar other
> sections should be (assuming student of the same general ability).
> I.e., if
> you gave the same exam to a different section (with the same general
> ability), then you would expect the means of the sections to fall
> within
> about 2.6 of each other.  As the difference gets larger, then you can
> be
> more confident that the two sections are different (e.g. better
> students,
> better teaching, they got the answers from previous sections, etc).
>
> Personally, I'd rather have them report the standard deviation -
> especially
> if that is what they say they are reporting!
>
> (This is a hunch. This is only a hunch.  If this had been actual
> knowledge,
> you would have been informed where to tune for further information ;-)

In case somebody wants to figure out what they present
as standard deviation here are the test data. These are
13 scores from one column:

58  157  195  147  131  79  183  170  158  155  119  108  194

My pocket calculator tells me: mean=142.61 and st.dev=42.31
The BlackBoard output is mean=142.61 and st.dev=14.25

The so-called standard deviation of the mean, would be
42.25 divided by the square root of 13 would be 11.73.
This is still not equal to 14.25.