Writing Resource:

Why Readability Measures Are Misleading

As any teacher of technical writing will tell you, readability formulas are hardly new. The simplest of these formulas relies on the one aspect of writing anyone can easily quantify—syllables in words and numbers of words in sentences. However, Flesch's Reading Ease score transformed that simple act of counting with an equation that appears vaguely terrifying:

Score = 206.835 – (1.015 X Average Sentence Length) – (84.6 X Average Syllables per Word).

If you're hankering after something a bit less numeric and more user-friendly, you might turn to the Flesch-Kincaid score, which correlates the Flesch Reading Ease score to American standards for reading at grade levels and to the estimated percentage of the U.S. adult population capable of reading at those grade levels. A score of 0-30 designated a document readable only by university graduates, while a score in the 90-100 range was readable by a fifth grader, and, by extension, readable by over 93% of American adults by Flesch's reckoning. Perversely, the higher a Flesch Reading Ease score, the easier the reading of the sentence or passage. In contrast, the lower scores on the Flesch-Kincaid indicate easier reading, with Dr. Seuss' Green Eggs and Ham scoring an improbable -1.3. This score suggests the ideal reader of Green Eggs and Ham is a fetus.

However, Flesch and Flesch-Kincaid scores can be challenging for even experts to compute without resorting to built-in software style checkers. As a result, researchers tried a simpler formula, now embodied in Gunning's FOG Index:

Grade Level = .4 (average sentence length + hard words of more than two syllables).

Flesch, Flesch-Kincaid, and the FOG scores are excellent at ensuring reading prompts in an experiment are all more or less comparable in their demands on readers' educations and brains. However, none of these formulas provide much in the way of insight into the challenges sentences throw at readers. For starters, the number of syllables in a word is seldom a reliable gauge of its difficulty. Specifically, praxis and baseball have the same number of syllables, but few fifth graders would have a nodding acquaintance with praxis, for all its two-syllable length.

Moreover, readability formulas are deceptive, as they rely on a relentless counting of syllables and words to arrive at a sense of how challenging a reader might find any sentence or document. Yet a brief sentence can prove more demanding on your readers' brains than a sentence of three or four times its length. Consider, for instance, a sentence that few high school students would find challenging to read, one drawn at random from Lynne Truss' Eats Shoots & Leaves, a paean to punctuation:

Using the comma well announces that you have an ear for sense and rhythm, confidence in your style and a proper respect for your reader, but it does not mark you out as a master of your craft.

Even though Truss begins with a gerund phrase, using the comma, acting as a grammatical subject, and the sentence runs to 38 words, the readability analytics give her sentence a thorough drubbing, with a Flesch score of 50.3, a Flesch-Kincaid of 15.7, and a FOG of 17.3. Put simply, the FOG Index predicts that only someone with a masters' degree could make sense of that sentence. However, Eats, Shoots & Leaves enjoyed the sort of best-seller status reserved for airport paperbacks. Either most of the book's readers understood it and enjoyed reading it—as the book's sales were generated initially by word of mouth—readability scores be damned—or people bought it without reading it. Given the track record of readability scores, one suspects millions bought and read it, as likely as the fact that few fetuses can read Green Eggs and Ham.


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