Footnote

a This idea is roughly based on the work of the 19th-century Italian economist Vilfredo Pareto and is also known as the Pareto principle. It involves the observation that often 80 percent of the results come from about 20 percent of the effort. It has been applied to many things, but here is a simple example: When a carpet is vacuumed, about 80 percent of the dirt picked up is likely from 20 percent of the carpet, namely, the high-traffic areas.