The Pint Slide Rule

By Henry Woodbury

This is a not a post about a beer gauge. It is a post about cognitive bias. To quote from the item itself: “[Jean] Piaget studied the tendency to focus attention on only one characteristic. In our case: beer height not volume!”

The gauge is the invention of engineer and physicist Chris Holloway. The Wall Street Journal Numbers Guy, Carl Bialik, explains:

Holloway has noticed that the typical pour in a pint glass is less than a pint. And since the widest part of the glass is at the top — nearly twice as wide as the bottom — leaving just the top half-inch of the glass unfilled costs the customer nearly 15% of the pint he’s paying for. So what may look trivial to bartenders and to drinkers, thanks to our tendency to focus on height rather than width when taking the measure of liquids, is a serious tavern injustice.

Readers of Edward R. Tufte will remember that one grave mistake in visualizing data is to show one-dimensional data with two-dimensional graphics:

There are considerable ambiguities in how people perceive a two-dimensional surface and then convert that perception into a one-dimensional number. Changes in physical area on the surface of a graphic do not reliably produce appropriately proportional changes in in perceived areas. The problem is all the worse when the areas are tricked up into three dimensions. (The Visual Display of Quantitative Information, 2001, p. 71.)

Holloway had the reverse challenge — the problem people have perceiving differences in volume. The gauge turns three-dimensional data into a one-dimensional series. It is a portable liquid measure.

I will add that despite the elegance of the Holloway’s concept, the gauge as is could use some design improvement. Holloway has elected to emphasis even numbers of ounces. I’m not sure this helps readability. There’s also no reason to have the 6 oz. label offset from the 6 oz. line. Just make the card a quarter inch longer or so. Nor is there reason for the key or the “Glass edge” and “Beer surface” labels. Instead, replace all of this extra text with a 16 oz. line aligned to the top of the glass that incorporates some minimal description. For example:

16 oz beer / 0% missing
15 oz / 7%
14 oz / 13%

etc.