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Educational Psychology: Assessment · Issues · Theory & research · Techniques · Techniques X subject · Special Ed. · Pastoral

In linguistics, the Gunning-Fog Index is a test designed to measure the readability of a sample of English text. The resulting number is an indication of the number of years of formal education that a person requires in order to easily understand the text on the first reading. That is, if a passage has a fog index of 12, it has the reading level of a U.S. high school senior. The test was developed by Robert Gunning, an American businessman, in 1952.[1]

The fog index is generally used by people who want their writing to be read easily by a large segment of the population. Texts that are designed for a wide audience generally require a fog index of less than 12.

### Calculating the Gunning-Fog Index

The Gunning-Fog index can be calculated with the following algorithm:

1. Take a full passage that is around 100 words (do not omit any sentences).
2. Find the average sentence length (divide the number of words by the number of sentences).
3. Count words with three or more syllables (complex words), not including proper nouns (for example, Djibouti), compound words, or common suffixes such as -es, -ed, or -ing as a syllable, or familiar jargon.
4. Add the average sentence length and the percentage of complex words (ex., +13.37%, not simply + .1337)
5. Multiply the result by 0.4

The complete formula is as follows:

((words/sentence) + 100 * (complex words/words)) * 0.4

While the index is a good indication of reading difficulty, it still has flaws. Not all multisyllabic words are difficult. For example, the word "asparagus" is generally not considered to be a difficult word, even though it has four syllables.

### Example

The following paragraph, from the Wikipedia article on "logorrhea", has a Gunning-Fog Index of 17.5.

The word logorrhoea is often used pejoratively to describe prose that is highly abstract and contains little concrete language. Since abstract writing is hard to visualize, it often seems as though it makes no sense and all the words are excessive. Writers in academic fields that concern themselves mostly with the abstract, such as philosophy and especially postmodernism, often fail to include extensive concrete examples of their ideas, and so a superficial examination of their work might lead one to believe that it is all nonsense.