In linguistics, the Gunning fog index is a test designed to measure the readability of a sample of English writing. The resulting number is a rough estimate of the number of years of formal education that a person requires in order to understand the text on a 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.
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.
- Texts that require a close-to-universal understanding generally require an index of less than 8.
The Gunning fog index can be calculated with the following algorithm:
- Take a full passage that is around 100 words (do not omit any sentences).
- Find the average sentence length (divide the number of words by the number of sentences).
- Count words with three or more syllables (complex words), not including proper nouns (for example, Djibouti), familiar jargon or compound words, or common suffixes such as -es, -ed, or -ing as a syllable.
- Add the average sentence length and the percentage of complex words
- Multiply the result by 0.4
The complete formula is as follows:
While the index is a good indication of reading difficulty, it still has limitations. 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.
Until the 1980s this index was calculated differently. The original formula demanded independent clauses to be counted as separate sentences in determining the total score.
Since the purpose of the index was to measure the clarity of ideation within sentences, it recognised that readers perceive independent clauses as complete thoughts. This changes the sentence index for some writers more than others.
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