a cumulative frequency is up to SOME point ... the problem is, WHAT is the
point
does it include THE point? i don't really see much (if any) difference
between (say we have a score scale that goes up to 50 and, 1 point is given
for each valid response) saying we have accumulated 53% to a score of 38
... or, the upper limit of 38.5 ... or for that matter, anywhere between 38
and (but not quite) 39 ... people can't get scores of decimal values anyway
an upper limit, by definition, is always a value that can't be achieved
At 06:08 AM 5/22/00 +0000, [EMAIL PROTECTED] wrote:
>Hi all,
>
>First up, the purpose I have at hand is to make interpolations for
>percentages of students who have achieved above a certain score on a
>test (where this score may lie between two discrete score points on the
>scale).
>
>It seems to me cumulative frequencies should be plotted at the exact
>upper limit of each interval. This is the only simple method that
>makes sense to me.
>
>However, it has been suggested by others in the context I'm dealing
>with that frequencies/percentages can alternatively be plotted at the
>mid-point of each interval, or even at the lower limit! Although I can
>understand plotting graphs at the mid-point for ease of representation,
>this hardly seems suited to making interpolations. This is because
>when you read off the graph at the upper limit of a given interval, you
>will (probably) have more cases than fell up to and including the
>interval itself. This is surely absurd, yet people seem to seriously
>believe it is a viable alternative.
>
>I'm really hoping for a good reference on this (preferably by a highly
>regarded author to make the case stronger :). Any comments, or refs?
>
>Thanks!
>
>Steve.
>
>
>Sent via Deja.com http://www.deja.com/
>Before you buy.
>
>
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Dennis Roberts, EdPsy, Penn State University
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