Automated Readability Index

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The Automated Readability Index (ARI) is a readability test designed to gauge the understandability of a text. Like the Flesch-Kincaid Grade Level, Gunning-Fog Index, SMOG Index, and Coleman-Liau Index, its output is an approximate representation of the U.S. grade level needed to comprehend the text.

Unlike the other indices, the ARI, along with the Coleman-Liau, relies on a factor of characters per word, instead of the usual syllables per word. Although opinion varies on its accuracy as compared to the syllables/word and complex words indices, characters/word is often easier to calculate, as the number of characters is more readily and accurately counted by computer programs than syllables.

[edit] Formula

To calculate the Automated Readability Index:

Divide the number of characters by the number of words, and multiply by 4.71. This is #1.
Divide the number of words by the number of sentences, and multiply by 0.5. This is #2.
Add #1 and #2 together, and subtract 21.43.

$ARI = 4.71 \left ( \frac{\mbox{characters}}{\mbox{word}} \right ) + .5 \left ( \frac{\mbox{words}}{\mbox{sentence}} \right ) - 21.43$