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.
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