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Flesch Reading Ease Formula is considered as one of the oldest and most accurate readability formulas. Rudolph Flesch, an author, writing consultant, and a supporter of the Plain English Movement, developed this formula in 1948. Raised in Austria, Rudolph Flesch studied law and earned a Ph.D. in English from the Columbia University. Flesch, through his writings and speeches, advocated a return to phonics. In his article, A New Readability Yardstick, published in the Journal of Applied Psychology in 1948, Flesch proposed the Flesch Reading Ease Readability Formula.

The Flesch Reading Ease Formula is a simple approach to assess the grade-level of the reader. It’s also one of the few accurate measures around that we can rely on without too much scrutiny. This formula is best used on school text. It has since become a standard readability formula used by many US Government Agencies, including the US Department of Defense. However, primarily, we use the formula to assess the difficulty of a reading passage written in English.

The specific mathematical formula is:

RE = 206.835 – (1.015 x ASL) – (84.6 x ASW)

ASL = Average Sentence Length (i.e., the number of words divided by the number of sentences)

ASW = Average number of syllables per word (i.e., the number of syllables divided by the number of words)

The output, i.e., RE is a number ranging from 0 to 100. The higher the number, the easier the text is to read.

• Scores between 90.0 and 100.0 are considered easily understandable by an average 5th grader.

• Scores between 60.0 and 70.0 are considered easily understood by 8th and 9th graders.

• Scores between 0.0 and 30.0 are considered easily understood by college graduates.

If we were to draw a conclusion from the Flesch Reading Ease Formula, then the best text should contain shorter sentences and words. The score between 60 and 70 is largely considered acceptable. The following table is also helpful to assess the ease of readability in a document:

90-100 : Very Easy
80-89 : Easy
70-79 : Fairly Easy
60-69 : Standard
50-59 : Fairly Difficult
30-49 : Difficult
0-29 : Very Confusing

Though simple it might seem, the Flesch Reading Ease Formula has certain ambiguities. For instance, periods, explanation points, colons and semicolons serve as sentence delimiters; each group of continuous non-blank characters with beginning and ending punctuation removed counts as a word; each vowel in a word is considered one syllable subject to: (a) -es, -ed and -e (except -le) endings are ignored; (b) words of three letters or shorter count as single syllables; and (c) consecutive vowels count as one syllable.