The Flesch–Kincaid readability tests are readability tests designed to indicate how difficult it is for a reader to understand a given passage of written English. The Plain English proponent, Rudolf Flesch devised the Reading-Ease test to evaluate the readability of an English text. Later, Flesch and the psychologist J. Peter Kincaid jointly developed the Grade Level test for the U.S. Navy.
The two tests of the readability of an English text are: (i) the Flesch Reading-Ease test and (ii) the Flesch–Kincaid Grade Level test. Each test of written-language use applies the same core measures (word length and sentence length) to a text, but each test gives different critical weight in evaluating the readability-measures of a text written in English. The results of the readability tests correlate approximately inversely: a text with a high-readability score on the Reading-Ease test should have a low-readability score on the Grade-Level test.
Understand readability scores
Each readability test bases its rating on the average number of syllables per word and words per sentence. The following sections explain how each test scores your file's readability.
Flesch Reading Ease test
This test rates text on a 100-point scale. The higher the score, the easier it is to understand the document. For most standard files, you want the score to be between 60 and 70.
The formula for the Flesch Reading Ease score is:
206.835 – (1.015 x ASL) – (84.6 x ASW)
where:
- ASL = average sentence length (the number of words divided by the number of sentences)
- ASW = average number of syllables per word (the number of syllables divided by the number of words)
Flesch-Kincaid Grade Level test
This test rates text on a U.S. school grade level. For example, a score of 8.0 means that an eighth grader can understand the document. For most documents, aim for a score of approximately 7.0 to 8.0.
The formula for the Flesch-Kincaid Grade Level score is:
(.39 x ASL) + (11.8 x ASW) – 15.59
where:
- ASL = average sentence length (the number of words divided by the number of sentences)
- ASW = average number of syllables per word (the number of syllables divided by the number of words)
Understand how languages affect readability scores
The languages that you use in a document can affect how your Office program checks and presents readability scores.
If you set up Word to check the spelling and grammar of text in other languages, and a document contains text in multiple languages, Word displays readability statistics for text in the last language that was checked. For example, if a document contains three paragraphs — the first in English, the second in French, and the third in English — Word displays readability statistics for the English text only.
For some European languages within an English document, Word displays only information about counts and averages, not readability.
Ref: https://support.microsoft.com/en-us/word/about-flesch-kincaid-readability-and-level-statistics
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