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Gunning's Fog Index

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I was reading a job profile for service engineers. One of the things that caught my eye was the assumption that they would have the reading ability of an average 16 year old. Now, all trainers want their material to match their students, but how on earth do you assess readability and adapt your style to match a given reading age?

Fortunately, it is possible to adapt your style to suit readers of varying age and education. You can even do it by numbers. Robert Gunning, an American business teacher has invented a technique for measuring readability and it will be very much worth your while learning how it works. Gunning's 'Fog Index' combines a word-length test and a sentence-length test, to assess the minimum age and educational level for which a text is suitable. This is the formula:

Fog Index = (Average sentence length + percentage of long words) x 0.4

And this is how you work it out for a given passage of text:
  1. Select a typical example of about 100 words.
  2. Count the number of words.
  3. Divide the number of words by the number of sentences. This gives you the average sentence-length.
  4. Count the number of long words in the sample (i.e. the number of words with more than three syllables).
  5. Add the average sentence-length to the number of long words.
  6. Multiply the result by 0.4 to get the Fog Index.

Gunning added the final step to make his index fit the grades of the U.S. school system. An index of 10 means that readers must have reached at least grade 10 if they are to follow the text easily. Roughly, it shows the minimum number of years of formal education required for sustained understanding and attention.

A Fox Index of 10 corresponds to readers who are about 15 years old, or who left school at that age. The range of 11-13 implies a senior school level, up to sixth form in the English system. The range 14-16 goes with a university education, or equivalent practice with fairly difficult books. 17 and 18 are marginal cases. A Fog Index of over 18 is too hard.

Let's look at some examples. First an extract from Ian Fleming's Dr. No:

Bond took the opportunity to glance at his watch. Ten o'clock. His eyes slid to the gun and holster on the desk. He thought of his fifteen years' marriage to the ugly bit of metal. He remembered the times its single word had saved his life - and the times when its threat alone had been enough. He thought of the days when he had literally dressed to kill when he had dismantled the gun and oiled it and packed the bullets carefully into the spring-loaded magazine and tried the action once or twice, pumping the cartridges out on to the bedspread in some hotel bedroom somewhere around the world.

Number of words: 109
Number of sentences: 6
Average sentence length: 18.2 words
Number of long words: 6 (6 per cent)
Fog Index = (18.2 + 6.0) x 0.4 = 9.7, Say 10 (reading age of 15).

Next let's look at an extract from a University Research Project:

The practitioner facilitates client catharsis but gives no space for the flow of autogenic insight which invariably follows if the client is not interfered with and encouraged to verbalise it. More complete recall, restructuring of the past with new evaluation, newly perceived connections between past and present: sustained catharsis will sooner or later give way to this spontaneous self-generated interpretation. Catharsis is incomplete without his flow of insight which it releases. The practitioner may cut it off simply by giving space only for the emotional discharge or, more fatally, by imposing his own interpretations and analyses on the client.

Number of words: 99
Number of sentences: 4
Average sentence length = 25 words
Number of long words = 21
Fog Index = (21 +25) x 0.4= 18.4 say 18 (reading age of 23).

A Fog Index of 18 suggests that the above extract was only just understandable by the people it was written for.

All this takes a bit of calculating so I have written a Reading Age Calculator program that does all the work for you. It uses a different algorithm to Gunning's Fog Index, but it comes up with very similar results. Level 3 members can also download a Mac version of this program. Not a member? Sign up here.

Incidentally, the reading age for this article is about 15 (Phew! that's a relief. I won't have to do a rewrite).


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