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RE: st: RE: Error Bars on Histogram or Bar Plots


From   Buzz Burhans <wsb2@cornell.edu>
To   statalist@hsphsun2.harvard.edu
Subject   RE: st: RE: Error Bars on Histogram or Bar Plots
Date   Mon, 02 Jun 2003 13:17:13 -0400

Agreed that we agree, mostly. But the premise that "lots of
people do this, so it is not discussable" is not one I sign
up to. Lots of people use pie charts, but when I can I try
to persuade audiences that it's a usually a bad idea. And
in this detail, and almost all others to do with graphics,
I stand on the shoulders of giants.

More pertinently, I gave three specific arguments
against this kind of error bar. I await counter-arguments.
I don't subscribe to the belief that just because others do something it is justified either. On the other hand, I don't presume to tell economists, or geographers, or anyone else, how their discipline should or shouldn't present data. I do recognize that there are conventions that differ in different disciplines , and may make more sense to the knowing than to the untutored in a particular discipline.

In any case, I don't wish to engage in a debate that has little merit. I expressed an opinion, made it clear that is an opinion, and that it is at least apparently shared by others in my or related disciplines. I further think that some of the arguments raised in the earlier course of the discussion of graphical representations of data were a bit pedantic and purist. I am of the opinion that the intent of graphic representations is to inform the reader, and I also expect the reader to be intelligent enough to invest some thought in making sense of them. I think that the paramount issue is remembering that the intent of a graphic should be to inform, in many cases a graphic can accomplish this better than text. in as much as these issues touch on error bars, in specific, I have the following thoughts regarding you arguments:


1. Their visual signal conflates two
distinct issues, estimate relative to
zero and estimate relative to error
bar.
Indeed, it does. While I have no interest in debating this, a full appreciation of the significance (i.e. meaningfulness, not '"statistical significance") of information presented by many graphics requires consideration of both of these "distinct issues". Sorry, but the error surrounding the statistic is as important a piece of evaluating the information as the statistic itself (again, in some / many cases). In some cases graphics without error bars of the type you portrayed may either misinform, because without error bars the relative 'statistical" significance of the bars is left unaddressed, or may at best leave one uninformed. My opinion is that if one is going to represent an estimate to me, I would prefer some information about the error at the same time, or I do not have enough information to critically asssess the information presented. So I am interested in both the magnitude of the data, i.e. relative to 0, and the variance. Only one or the other leaves me either ill informed, or as sometimes happens, misinformed when data is presented without error information, and that sometimes does occur unfortunately. In fact, it sometimes occurs intentionally, or in some cases it occurs in the company of sloppy or non-rigorous thinking.



2. Suppressing the bottom part of the
error bar obliges the concerned user
to guess mentally where it ends. That's
very poor.
True, it imposes the necessity of some mental effort by the reader in order to interpret the information as fully as possible. Sometimes, I find I must actually read the paper as well !! instead of just looking at the graphics. If there is value in graphic representation of data (which I believe there is), then it should be as well presented as possible, but even the best graphic presentation of the data does not remove the effort a reader must make to understand it, nor does it absolve the reader or the author from the need to make an intellectual effort to effectively represent and understand the information. In the case of the half bars, an imperfect but genuine representation may require more effort from the reader, but it is preferable to providing even less information by omitting the bars altogether.




3. Such graphs take a lot of ink to show
relatively little information.
Au contraire! A point I suppose, of graphic representation, is to efficiently represent a substantial amount of information in a fairly consolidated manner. While you may prefer to be partially informed, I prefer to provide, and I absolutely insist in being provided, more complete information that facilitates a more informed assessment of the information in the graphic. I think that the fact that so much work published in biological sciences journals uses some type of error bar suggests that there may be a functional basis for doing so.



Something like a dot chart seems to me
far preferable. Cleveland's work is relevant here.
I suppose my ignorance is showing, but I don't know who Cleveland is...and I have no quarrel with your liking dot charts. They don't particularly appeal to me, and their use is somewhat minimal in the literature I read, but that doesn't make them unimportant, or more importantly, uninformative. However, there are many of us for whom the issues we are discussing are better , and in my opinion, validly, represented in other ways, at least frequently.

Buzz Burhans



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