Flesch’s Quick Rule-of-thumb Yardstick

By Nirmaldasan


In The Art Of Plain Talk, Rudolf Flesch says that simple language consists of ‘short sentences, few affixes, and many personal references’.  The average words per sentence (W), percentage of affixes (A) and percentage of personal references (P) are strung into a complicated expression: Difficulty score = (0.1338 * W) + (0.645 * A) – (0.0659 * P) – 0.75. Scoring system: up to 1 (very easy, 5th grade); 1 to 2 (easy, 6th); 2 to 3 (fairly easy, 7th); 3 to 4 (standard, 8th to 9th); 4 to 5 (fairly difficult, 10th to 12th); 5 to 6 (difficult, 13th to 16th); and 6 or more (very difficult, college graduate).

But in a postscript, Flesch presents a Quick Rule-of-thumb Yardstick (QRY): Difficulty score = [(A – P) / 2] + W. Scoring system: up to 13 (very easy), 13 to 20 (easy), 20 to 29 (fairly easy), 29 to 36 (standard), 36 to 43 (fairly difficult), 43 to 52 (difficult) and 52 or more (very difficult). But if we take a sample of 50 words instead of 100, then the calculation becomes simpler. Let ‘a’ and ‘p’ be the affixes and personal references in a sample of 50 words; and ‘w’, the average number of words per sentence. Then, difficulty score = w + a – p.

Affixes are extremely hard to spot, but Flesch gives a helpful list of affixes in the appendix. Personal references are easy to locate: names of people, personal pronouns that refer to people and a finite list of human-interest words.

Let’s apply the QRY on the following 50-word paragraph taken from a longer sample analysed by Flesch (the personal references are in capitals and the affixes are in brackets):

“WE shall plan, (with)in each countr(y) and (be)tween countr(ies), for more jobs and for mak(ing), trad(ing) and us(ing) more goods. (Al)so, WE shall plan to do (a)way with all ways of treat(ing) the trade of some countr(ies) bett(er) than that of others, and to low(er) tariffs and other trade barr(iers).”

Number of sentences = 2

Number of words = 50

w = 50/2 = 25

a = 14

p = 2

Difficulty score = w + a – p = 25 + 14 – 2 = 37 (fairly difficult)

For a reliable assessment, the QRY must be applied on at least 10 samples of 50 words each. “Some readers, I am afraid,” writes Flesch, “will expect a magic formula for good writing and will be disappointed with my simple yardstick. Others, with a passion for accuracy, will wallow in the little rules and computations but lose sight of the principles of plain English. What I hope for are readers who won’t take the formula too seriously and won’t expect from it more than a rough estimate.”


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