John McElroy, author of Techniques For Clear Informative Writing (1950), developed the Fog Count (FC) to measure reading ease. In a single-sentence sample, Easy words (E) of one or two syllables are counted once and hard words (H) of three or more syllables are counted thrice. Thus FC = E + 3H. I came across a fine description of this readability formula in Guide For Air Force Writing, published by the U.S. Department of Air Force, 1954 — full text available in the Hathi Trust Digital Library: http://www.hathitrust.org
The formula does not treat all polysyllables as hard words. Names of persons, places, months and days are considered to be easy. Michelangelo, Mesopotamia, September and Wednesday, in spite of their being polysyllables, are each counted once. Abbreviations and numbers are also treated as easy words. So, UNESCO and 3.141 are each counted once. Moreover, compound names of persons with common titles are treated as single names. So, President Barack Obama gets a count of only one.
The best way to understand a formula is to apply it. Please re-read the first sentence of this article. It has 19 words, but only 13 idea units:
John McElroy — 1 (compound name); author —1 (disyllable); of — 1 (monosyllable); Techniques For Clear Informative Writing — 1 (compound name); (1950) — 1 (number); developed — 3 (polysyllable); the — 1 (monosyllable); Fog Count — 1 (compound name); (FC) — 1 (acronym); to — 1 (monosyllable); measure — 1 (disyllable); reading — 1 (disyllable); ease — 1 (monosyllable).
Add the counts to get FC. In this case the fog count is 15. The Air Force Manual says: “To write within the reading grasp of the average man, keep the average Fog Count under 25.” The manual also adds: “To write within the comfortable grasp of the average man, hold the average Fog Count at 20 or under.”
McElroy’s Fog Count may be converted into a grade level. If the FC is 20 or more, then Grade Level (GL) = FC / 2. Otherwise, GL = (FC – 2) / 2. Since the first sentence of this article has an FC of 15, its GL = (15 – 2) / 2 = 13 / 2 = 6.5.
In 1975 J.P. Kincaid, R.P. Fishburne, R.L. Rogers and B.S. Chissom suggested the New Fog Count in a report titled ‘Derivation of new readability formulas (Automated Readability Index, Fog Count and Flesch Reading Ease Formula) for Navy enlisted personnel’. According to the New Fog Count, GL = (FC – 3) / 2. For the opening sentence, GL = 6.
The grade levels of McElroy’s Fog Count and the New Fog Count differ by 0.5 in the lower grades and 1.5 in the higher grades.
New Fog Count Grading (NFCG): If we take a sample of five sentences, we get a fine simplification. Let IU5 be the number of idea units in five sentences; and AHS, the average number of hard words per sentence. Now, NFCG = (IU5 / 10) + AHS – 1.5.
The first sentence has 13 idea units. So IU5 = 5 x 13 = 65. As there is only one hard word, AHS = 1. So, NFCG = (65 /10) + 1 – 1.5 = 6.5 – 0.5 = 6. For a better estimate, we need to apply the NFCG on five sentences.
The oft-repeated caution with regard to readability measures is always worth reiterating. The Air Force Manual says: “Low Fog Count does not guarantee clear meaning; high Fog Count does not always create reading difficulty.” The same is true of New Fog Count Grading, too.