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The Gunning Fog Index Readability Formula, or simply called FOG Index, is
attributed to American textbook publisher, Robert Gunning, who was a graduate from
Ohio State University. Gunning observed that most high school graduates
were unable to read. Much of this reading problem was a writing problem. His opinion
was that newspapers and business documents were full of “fog” and unnecessary
complexity.
Gunning realized the problem quite early and became the first to take the
new readability research into the workplace. Gunning founded the first consulting firm
specializing in readability in 1944. He spent the next few years testing and working with
more than 60 large city daily newspapers and popular magazines, helping writers and
editors write to their audience.
In 1952, Gunning published a book, The
Technique of Clear Writing and created an easy-to-use Fog Index.
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| The Gunning’s Fog Index (or FOG) Readability Formula
Step 1: Take a sample passage of at least 100-words and count the number of exact
words and sentences.
Step 2: Divide the total number of words in the sample by the number of sentences
to arrive at the Average Sentence Length (ASL).
Step 3: Count the number of words of three or more syllables that are NOT (i)
proper nouns, (ii) combinations of easy words or hyphenated words, or (iii) two-syllable
verbs made into three with -es and -ed endings.
Step 4: Divide this number by the number or words in the sample passage. For
example, 25 long words divided by 100 words gives you 25 Percent Hard Words (PHW).
Step 5: Add the ASL from Step 2 and the PHW from Step 4.
Step 6: Multiply the result by 0.4.
The mathematical formula is:
Grade Level = 0.4 (ASL + PHW)
where,
ASL = Average Sentence Length (i.e., number of words divided by the number of
sentences)
PHW = Percentage of Hard Words
The underlying message of The Gunning Fog Index formula is that short sentences
written in Plain English achieve a better score than long sentences written in complicated
language.
The ideal score for readability with the Fog index is 7 or 8. Anything above 12
is too hard for most people to read. For instance, The Bible, Shakespeare and Mark Twain
have Fog Indexes of around 6. The leading magazines, like Time, Newsweek, and the
Wall Street Journal average around 11.
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| Though considered as an accurate Readability Formula, The Gunning Fog Index
Formula has some unnoticeable flaws. For example, it discounts that not all multi-syllabic
words are difficult.
The Fog Index has also undergone important changes to enable computerization of
this formula, after facing different opinions among scholars about counting
independent clauses as separate sentences.
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