Monday, 3 January 2022

Calculating the Gunning Fog Index

While I have written about the Gunning (1968) fog factor before (here), it is worth taking a deeper look. American Publishing magnate Robert Gunning came up with a number of key measures and hints for better writing in 1952, along with a measure for overly dense writing called the Gunning Fog Index (1968). 

The fog index is a bit complicated to work out, but is calculated as follows:

    1. Take a 100-ish word sample of our writing. Count the number of words and sentences
    2. Divide the word count by the number of sentences. This gives us our average sentence length
    3. Complex words: count the number of words with three or more syllables, NOT including proper nouns; jargon; hyphenated words, or two syllable verbs made into three with -es, -ing, or -ed suffixes
    4. Divide the complex word total by the number of words in the sample. This is the percentage of hard words
    5. Add the average sentence length from (2) and the percentage of hard words from (4)
    6. Lastly, multiply the result by 0.4. This gives us our fog factor.

              For example, the following passage is 100 words:

              The cases which I used in this research were not seen by the students as being evenly engaging. The Lance Armstrong case apparently had ‘trouble’ (Bruner, 1990; “The Lance Armstrong case is really controversial”; NNF), legitimacy (Bruner, 1990; “easily the most interesting one - and it was like; it wasn't a challenge. I didn't have to think back or anything, to answer the questions”; MF, 11.07) and a satisfactory ending (Heidegger, as cited by Lewis, 2011, p. 206; “everyone was quite shocked at the lengths he went to, and like the things that weren't so obvious... Yeah, like the things that obviously people already knew, but the end they went into quite a bit of detail all the other things, on the negative side out of it all, and how he treated everyone. Yeah, like how much effort he put into cheating. It was crazy”; MF, 07.39). It contained the requirements of a good story, flowed for the students (i.e., well-written; “The Lance Armstrong case structure was easier; it was structured better” NF).

              This is 173 words, and 6 sentences, giving 28.8 words per sentence. There are 8 complex words which are not jargon, extended verb forms or proper nouns. The percentage of hard words is 4%. We add the two numbers (total of 33) then multiply by 0.4. This provides and overall fog factor of 13, or, in the US, university level. A little high, but not too terrible for academic writing.

              it is easy to see that reducing the sentence length will reduce the fog factor. I think this is one of the useful checks that we can do on our writing. There is even a website that allows us to run our own checks (here; My Byline Media, 2021). We simply enter our writing, click "calculate" and it will give us a total. Though please note that the site calculates the paragraph above at 9.3 - so 9th grade level in the US. Hmm. 

              Other writers, such as Bogert (1985) have suggested that the fog factor measurement is too simplistic, given that it only focuses on sentence and word length. However, even Bogert is charitable enough to allow that the fog index was "designed only as [a predictor] of reading ease" (1985, p. 9); it is a "tool, not a rule" (p. 9). 

              Still, checking our own writing is always useful to do, and to reflect on. 



              • Bogert, J. (1985). In Defense of the Fog Index. The Bulletin of the Association for Business Communication, 48(2), 9–12.
              • Gunning, R. (1968). The technique of clear writing (Rev. ed.). McGraw-Hill.
              • My Byline Media (2021). The Gunning’s Fog Index (or FOG) Readability Formula.
              • Wikipedia (2021). Gunning fog index.

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