The Coleman–Liau index is a readability test designed by Meri Coleman and T. L. Liau to gauge the understandability of a text.
Although opinion varies on its accuracy as compared to the syllable/word and complex word indices, characters are more readily and accurately counted by computer programs than are syllables.
The Coleman–Liau index was designed to be easily calculated mechanically from samples of hard-copy text.
As an example, we shall use the abstract from Coleman and Liau's original 1975 paper introducing the index: Existing computer programs that measure readability are based largely upon subroutines which estimate number of syllables, usually by counting vowels.
Both predictors can be counted by an optical scanning device, and thus the formula makes it economically feasible for an organization such as the U.S. Office of Education to calibrate the readability of all textbooks for the public school system.h The abstract contains 5 sentences, 119 words, and 639 letters or digits; L is 537 and S is 4.20 obtained by the formulas: L = (Letters ÷ Words) × 100 = (639 ÷ 119) × 100 ≈ 537 S = (Sentences ÷ Words) × 100 = (5 ÷ 119) × 100 ≈ 4.20 Therefore, the abstract is at a grade level of 14.5, or roughly appropriate for a second-year undergraduate.