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, Fry Readability Formula, and Coleman-Liau Index, it produces an approximate representation of the US grade level needed to comprehend the text.

The formula for calculating the Automated Readability Index is given below:

$4.71 \left (\frac{\mbox{characters}}{\mbox{words}} \right) + 0.5 \left (\frac{\mbox{words}}{\mbox{sentences}} \right) - 21.43$

where characters is the number of letters, numbers, and punctuation marks, words is the number of spaces, and sentences is the number of sentences. Sentences were counted by hand as each text was typed.

As a rough guide, US grade level 1 corresponds to ages 6 to 8. Reading level grade 8 corresponds to the typical reading level of a 14 year-old US child. Grade 12, the highest US secondary school grade before college, corresponds to the reading level of a 17 year-old.

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 faster to calculate, as the number of characters is more readily and accurately counted by computer programs than syllables. In fact, this index was designed for real-time monitoring of readability on electric typewriters.[1]

Notes

1. ^ Senter, R.J.; Smith, E.A. (November 1967). Automated Readability Index.. Wright-Patterson Air Force Base. p. iii. AMRL-TR-6620. Retrieved 2012-03-18.