The Gunning FOG index is a test designed to measure the readability or density of a sample of English writing based on sentence length.
The general rule in business report writing, is to keep sentences succinct, short and compact at around 15-20 words each.
The test is based on an estimate which is calculated as to the number of years of formal education that a person needs in order to understand the sample on the first read through.
The test was developed by Robert Gunning, an American businessman, in 1952.
The Fog index is mainly used by writers who target a wide demographic and want their writing to be understood easily.
Based on the test, in order for text to reach an audience with a close-to-universal understanding, the index needs to be less than 8.
How To Calculate 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), familiar jargon or compound words, or common suffixes such as --es, -ed, or --ing as a syllable
4. Add the average sentence length and the percentage of complex words
5. Multiply the result by 0.4
While the index is a good indication of reading difficulty, it still has limitations. Not all multi-syllabic words are difficult. For example, the word "asparagus" is generally not considered to be a difficult word, even though it has four syllables. Additionally, a short sentence can be crammed full of complex words.
The following paragraph, from the Wikipedia article on "logorrhoea", has a gunning Fog Index of 16.6.
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 visualise, 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.
The Fog Index Table
The following table is an indication of readability based on the average number of words in sentences within a passage of writing.
If a sentence has eight words or less, it is deemed 'easy to read'. Additionally, an average reader would have no difficulty in reading a sentence of 17 words in length. However, a sentence comprised of 29 words or more, is considered 'difficult to read'.
A variety in sentence length will generally increase a reader's interest.
# of Words & Readability
8 or less = Very easy
11 = Easy
14 = Fairly easy
17 = Standard
21 = Fairly difficult
25 = Difficult
29 = Very difficult
Resources -- External Links:
* Fog Index Calculator (http://simbo.madpage.com/Fog/) A simple web page to cut-and-paste text and then calculate Fog Index
* Online Fog Index Calculator (http://www.online-utility.org/english/readability_test_and_improve.jsp)
Suggestions as to how to improve readability, different measurements.
* Readability Calculator (http://www.sigmalist.com/SigmaMathTool/FogIndex.aspx) Clear and easy.