Much as I love Tufte’s work, he does base much of what he says on aesthetics and his (considerable) intuition. Another approach, of which Cleveland has been an important pioneer, is to draw conclusions based on empirical testing of which forms of data representation are most effective. In their 1991 paper Displaying Proportions and Percentages, Ian Spence and Stephan Lewandowsky compared the efficacy of bar charts, divided bar charts, pie charts and tables. Their focus was on asking three different types of questions about sets of data containing between four and seven data points (A, B, C,…). These were to determine which was greater A or B, A or B+C and A+B or C+D (note that the data was not necessarily presented in that order). They concluded that subjects found it harder to make all three judgements with a table than with either bar charts or pie charts.

One important point here though is that the subjects of the experiment were deliberately selected from groups that were not scientists, statisticians or experts at reading charts and data. So, these conclusions are only really applicable for presenting information to a general audience. Still, that is an important domain to consider.

Table: Cases of Marmalade Sold

         US       UK
Jan    4,000     9,800
Feb    6,000    11,000
Mar    6,400    11,500

Love the karma/dogma line…I plan to steal it!

Sean.

Thank you for sharing. I would feel more sense of closure about Cleveland and bar charts at this moment myself . . . except I’ve written myself into a corner.

Ironically, I noticed today that an Excel app that I’d set up years ago is still being used to report results to my organization using bar charts with, well, partial bars. Clearly, I am guilty of an engineering compromise, made long before I’d stumbled onto this website and learned about how to make dot plots using Excel, or read Cleveland to understand why they are better. My coworkers seem quite content with these bar charts — and it’s not like the abstract index they represent is something one can actually count or measure with a ruler, anyway. No harm, no foul?

Suddenly I’m appreciating your shades of gray, Sean!

What to do when one’s karma runs over one’s dogma? I guess that my best way of atoning is to try to update my less than ideal bar charts with a dot plot. If that radical idea isn’t accepted, my fall-back recommendation would be to show the whole bars, even though the underlying data is about 90% locational and 10% length (or retinal position), and showing the whole bars will hurt the resolution somewhat.