Many formulae have been produced from readability research for assessing the readability of a piece of writing. Some are long and complex, but one simple formula is the Gunning Fog Index. It gives a good approximation to the readability scores produced by more complex formulae. We suggest that you try using this formula on a piece of your own writing. Use a chunk of text from a report, essay, article, laboratory report, or any other continuous prose on an informative subject. It is important, however, to realise before you use it that this formula is intended only as a rapid, and fairly crude, estimate of readability. The results do not confirm, or deny, the laurels of good style to the writer.
The 'Fog Index' was first published by Robert Gunning in 1952.
Here are the steps for the calculation:
1. Find the average number of words per sentence. Use a sample at least 100 words long. Divide total number of words by number of sentences. This gives you average sentence length.
2. Count the number of words of three syllables or more per 100 words. Don't count: (a) words that are capitalized; (b) combinations of short easy words-like 'bookkeeper'; (c) verbs that are made three syllables by adding 'ed' or 'es'-like 'created' or 'trespasses'.