Robert Gunning created the Fog Index in 1952. This formula measures the years of schooling (1 to 17+) required to comprehend specific texts. In his books The Technique Of Clear Writing and How To Take The Fog Out Of Writing, he discusses principles that help writers create clear text.
The Plain Fog Index (PFI) is an attempt to explain Gunning’s formula in plain English and plain mathematics. The formula consists of two parts: Sentence Fog (SF) and Word Fog (WF).
PFI = SF + WF.
First, let us calculate SF. Take a sample of four sentences and count the number of words (W). SF = W/10.
Next, the WF. Take a sample of forty words. Count the number of hard words (H), which are words of more than two syllables excluding capitalized words, easy compound words and disyllabic verbs made trisyllabic by adding ‘-es’ or ‘-ed’.
Therefore, PFI = (W/10) + H.
Let us test this formula on the following passage, reproduced from William DuBay’s Smart Language:
“Robert Gunning was a graduate of Ohio State University. In 1935, he entered the field of textbook publishing. In the mid-1930s, educators were beginning to see high school graduates who were not able to read. Gunning realized that much of the reading problem was a writing problem. He found that newspapers and business were full of ‘fog’ and unnecessary complexity.”
In this sample, there are five sentences and 60 words. But we need only the first four sentences and the first forty words. W = 47 and H = 3. PFG = (W/10) + H = 4.7 + 3 = 7.7 years of schooling.