Zipf's Law

A statistical observation that in any natural language corpus, the frequency of a word is inversely proportional to its rank in the frequency table. The most common word occurs roughly twice as often as the second most common, three times as often as the third, and so on. In assistive technology, Zipf's Law is foundational to word prediction and word completion systems, because it means a small set of high-frequency words accounts for a large proportion of everyday text. This allows predictive text systems to offer useful completions after just one or two typed characters, significantly reducing input effort for users with motor impairments.