On Wed, 15 Dec 1999, Dr. Rick Froman (quoting himself) wrote:
>
> "If the author is to be blamed, it is not for drawing the conclusion
> that there was a significant difference between the groups but for
> failing to mention that the difference was not due to differences in
> central tendency but to differences in the distributions."
>
> Instead of saying things like, "a significantly lower sleepiness
> score was obtained in the experimental group" as the author does
> on p. 103 in the results section, the results should have been
> discussed in terms of differences in the distributions. Of course,
> this point is illustrated in the included graphs but some discussion
> of it in the text would have been better.
I think I'm going to have to retract some of my earlier comments after
checking my old but cherished copy of Siegel: Non-Parametric
Statistics. Rick may have to do the same (swallow your pride, not your
gum, Rick). The Mann-Whitney is used to test whether independent
groups have been drawn from the same population, but since you can
test directional hypotheses (Pop A is larger or smaller than B), then
it's defensible to conclude after a significant M-H that the
population chewing gum have lower sleepiness scores than the
population not chewing. The correspondence of medians is just a
curiosity--it's the entire distribution that counts. So I think that
Hodoba is ok on this point after all.
Rick continues
> I think the different distributions produced are very
> interesting findings in their own right that should have been
> discussed more fully in the article. I hope this response is easier
> to chew on than a three hour old stick of gum.
But before we get too excited about this, I've also noticed that the
experimental group was run (chewed?) at the end of September, while
the control group was run at the end of April/beginning of May (see p.
102, just before Experiment II). He may be studying spring fever
rather than gum-chewing.
-Stephen
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Stephen Black, Ph.D. tel: (819) 822-9600 ext 2470
Department of Psychology fax: (819) 822-9661
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