Sunlight or or info "lite"?

A good vizualisation illuminates and stimulates, but a bad one is just about worse than fluff, little substance and lots of eyecandy, if you're lucky. I recently saw a visualisation on a web page which purports to display statistical information about funds 'earmarked' by the US Office of Management and Budget. This viz uses the 'Many Eyes' toolkit created by the excellent crew at the Visual Communication Lab at IBM research. Many Eyes provides a basic toolkit for creating visualisations of all kinds - you just have to plug in some data and with a bit of fiddling, off you go with an interactive visualisation. It's pretty neat. The folks at Sunlight Labs have used Many Eyes to create a vizualization that maps these OMB earmarks to circles of various areas and colors. The resulting charts, which look like very pretty Ishihara Cards, may be aesthetically pleasing, but have problems.

The main one is that the human visual processing system is pretty poor at discriminating numerical values from areas, even ones that are symmetrical, like these circles. People tend to consistently under-report that value that is associated with an area of a given size. Take the two circles in this picture for example. The smaller circle is 20 units wide, and the larger one is 50 units wide. (The units are arbitraty - we could call them pixels, or even 'meow-meows'.) For making a comparative judgement about the costs associated with various expenditures we might ask, "How many smaller circles fit into the bigger one?" Our visual intuition consistently misleads us. People tend to estimate about 4-5 circles.

But a bit of simple geometry can illuminate the issue further. Taking the radii of the circles, we can work out the exact areas that are encoded. Since area=pi (π) times the radius squared, we can calculate the actual area of each circle. The smaller one is: 10 x 10 x 3.14159, or about 314 pixels and the larger one is 50 x 50 x 3.14159 or about 1963. dividing 1963 by 314, we can see that in fact, about 6.25 smaller circles can fit into the larger one.

Thus for a precise reading, relying on area to make an accurate judgement is not a good idea, and so designers of charts and graphics should avoid this pitfall. It could be argued that the folks at Sunlight Labs were generally trying to give an impression of a constellation of values, but the resulting scattershot image makes it difficult to make accurate comparisons and moreover forces the viewer to "hunt" for individual items of interest, such as States.

A better alternative for the purpose of accurate comparisons, rather than just impressions, which would avoid all of these problems would be the humble histogram...