Using Readability Formulas

A woman looking at distant, unreadable text
Can you read the text?.

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Any readability formula is one of many methods of measuring or predicting the difficulty level of text by analyzing sample passages.

A conventional readability formula measures average word length and sentence length to provide a grade-level score. Most researchers agree that this is "not a very specific measure of difficulty because grade level can be so ambiguous" (Reading to Learn in the Content Areas, 2012). See Examples and Observations, below.

Five popular readability formulas are the Dale-Chall readability formula (Dale & Chall 1948), the Flesch readability formula (Flesch 1948), the FOG index readability formula (Gunning 1964), the Fry readability graph (Fry, 1965), and the Spache readability formula (Spache, 1952).

Examples and Observations:

"Because researchers have been examining readability formulas for almost 100 years, the research is comprehensive and reflects both the positive and negative aspects of formulas. Essentially, research firmly supports that sentence length, and word difficulty provide viable mechanisms for estimating difficulty, but they are imperfect. . . .
"As with many tools that work with normally developing readers, readability formulas may require some tweaking when the target population includes struggling readers, learning-disabled readers, or English language learners. When readers have little or no background knowledge, readability formula results may underestimate the difficulty of the material for them, particularly for English language learners." (Heidi Anne E. Mesmer, Tools for Matching Readers to Texts: Research-Based Practices. The Guilford Press, 2008)

Readability Formulas and Word Processors

"Today many widely used word processors offer readability formulas along with spell checkers and grammar checkers. Microsoft Word provides a Flesch-Kincaid Grade Level. Many teachers use the Lexile Framework, a scale from 0 to 2000 that is based on average sentence length and average word frequency of texts found in an extensive database, the American Heritage Intermediate Corpus (Carroll, Davies, & Richman, 1971). The Lexile Framework circumvents the need to perform one's own calculations." (Melissa Lee Farrall, Reading Assessment: Linking Language, Literacy, and Cognition. John Wiley & Sons, 2012)

Readability Formulas and Textbook Selection

"There are probably more than 100 readability formulas currently in use today. They are widely used by teachers and administrators as a way of predicting if a text is written at a level appropriate for the students who will use it. While we can say with relative ease that readability formulas are fairly reliable, we need to be cautious in using them. As Richardson and Morgan (2003) point out, readability formulas are useful when textbook selection committees need to make a decision but have no students available to try out the materials on, or when teachers want to assess materials that students may be asked to read independently. Basically, a readability formula is a quick and easy way to determine the grade level of written material. However, we must remember that it is only one measure, and the grade level obtained is only a predictor and thus may not be exact (Richardson and Morgan, 2003)." (Roberta L. Sejnost and Sharon Thiese, Reading and Writing Across Content Areas, 2nd ed. Corwin Press, 2007)

The Misuse of Readability Formulas as Writing Guides

  • "One source of opposition to readability formulas is that they are sometimes misused as writing guides. Because formulas tend to have just two major inputs—word length or difficulty, and sentence length—some authors or editors have taken just these two factors and modified writing. They sometimes end up with a bunch of short choppy sentences and moronic vocabulary and say that they did it because of a readability formula. Formula writing, they sometimes call it. This is a misuse of any readability formula. A readability formula is intended to be used after the passage is written to find out for whom it is suitable. It is not intended as a writer's guide."
    (Edward Fry, "Understanding the Readability of Content Area Texts." Content Area Reading and Learning: Instructional Strategies, 2nd ed., edited by Diane Lapp, James Flood, and Nancy Farnan. Lawrence Erlbaum, 2004)
  • "Don't bother with the readability statistics. . . . The averages of sentences per paragraph, words per sentence, and characters per word have little relevance. The Passive Sentences, Flesch Reading Ease, and Flesch-Kincaid Grade Level are computed statistics that don't accurately assess how easy or hard the document is to read. If you want to know whether a document is hard to understand, ask a colleague to read it." (Ty Anderson and Guy Hart-Davis, Beginning Microsoft Word 2010. Springer, 2010)

Also Known As: readability metrics, readability test

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Your Citation
Nordquist, Richard. "Using Readability Formulas." ThoughtCo, Aug. 28, 2020, Nordquist, Richard. (2020, August 28). Using Readability Formulas. Retrieved from Nordquist, Richard. "Using Readability Formulas." ThoughtCo. (accessed March 22, 2023).