Jim Lemon wrote:
On 12/04/2009 12:56 AM, Frank E Harrell Jr wrote:
Bar charts with error bars are far inferior to dot charts and other
types of displays. One of many problems is demonstrated if you draw a
bar chart displaying temperature in F then re-draw it on the degrees C
scale. See http://biostat.mc.vanderbilt.edu/DynamitePlots for much
more information. The error bars lull us into an assumption that
symmetric confidence intervals are OK, among other things.
Frank
I could promote the "dispersion" function as capable of displaying
asymmetric confidence intervals on whatever location indicators you
prefer (have a look at the second example for "hierobarp" for example),
but the problem is deeper than that. The point of graphic displays is to
convey information to someone else, not to tell ourselves what we
already know. Do people cheat with pie charts, bar charts, etc? Sure,
and we could cheat with dot charts, too. Graphic displays are typically
shown to an audience that knows less about the topic than the presenter
in the hope that an obscure relationship will be clarified. If I am
presenting to colleagues in my field, I will use much more informative
and difficult to understand graphics than if I am summarizing the same
results to the general public. If both groups leave the respective
presentations enlightened, I have done well.
Jim
Jim I'll just add that in many cases the best graphics are just as easy
or easier to read and understand than bad graphics. And dot charts have
a much less chance of being misleading than bar and pie charts,
especially if more data characteristics or the data themselves appear.
Frank
--
Frank E Harrell Jr Professor and Chair School of Medicine
Department of Biostatistics Vanderbilt University
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