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st: Re: statalist-digest V4 #4814


From   Dirk Enzmann <dirk.enzmann@uni-hamburg.de>
To   statalist@hsphsun2.harvard.edu
Subject   st: Re: statalist-digest V4 #4814
Date   Fri, 08 Mar 2013 00:12:56 +0100

David,

In principle, I agree.

Which reminds me to the old Radio Yerevan jokes, which always started with "Is it true that ...?" to which the answer was "In principle yes, but ..."

My "but" in this case would be that examining overlap to examine significance is possible if you do not apply the too simplistic "no overlap" rule. This I tried to stress because I think that it is very useful to add confidence intervals to graphic displays of mean differences (also to avoid a substantive interpretation of non significant differences).

To which, of course, the without any doubt correct reaction will be "But if exact p values are desired they should also be presented."

And: You are right that readers could misinterpret my quote from Schenker and Genlemen (2001). Thanks for clarifying.

Dirk

Wed, 6 Mar 2013 20:58:55 -0500 David Hoaglin<dchoaglin@gmail.com> wrote:

Subject: Re: st: confidence intervals overlap [was: positive interaction - negative covariance]

Dirk,

I don't agree that that warning goes too far.  It is consistent with
the advice given by both Schenker and Gentlemen (2001) and Cumming and
Finch (2005).

Readers who are not familiar with the paper by Schenker and Gentleman
might interpret the material that you quoted as an endorsement of
judging significance according to overlap.  In fact, the quoted
material appears at the beginning of the first paragraph of the
introduction.  That paragraph concludes as follows: "Over a number of
years, however, we have tried to discourage the practice of examining
overlap to assess significance among authors of articles that we have
reviewed, because the procedure can lead to mistaken conclusions."

The paper by Cumming and Finch is highly useful.  I'm glad you called
attention to it.  As they make clear, their "Rules of Eye" are
intended to help readers when an article includes, for example, a
figure that shows confidence intervals for the means of two
independent samples and does not show a confidence interval for the
difference between those means.  On page 174 they say, "By analogy
with rules of thumb, our rules are intended to be useful heuristics or
pragmatic guidelines,  They are not intended to be numerically exact
or to replace statistical calculations: If exact p values are desired,
they should also be presented."

David Hoaglin

========================================
Dr. Dirk Enzmann
Institute of Criminal Sciences
Dept. of Criminology
Rothenbaumchaussee 33
D-20148 Hamburg
Germany

phone: +49-(0)40-42838.7498 (office)
       +49-(0)40-42838.4591 (Mrs Billon)
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email: dirk.enzmann@uni-hamburg.de
http://www2.jura.uni-hamburg.de/instkrim/kriminologie/Mitarbeiter/Enzmann/Enzmann.html
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