The Fog Index

The Fog Index is a readability formula that helps you to determine, to a degree, what kind of audience a specific magazine targets based on the average sentence length and number of three syllable or more words that appear in their articles. It also does the same job on all other types of prose, and basically tells you how 'intelligent' your average reader is going to need to be to understand and appreciate what you're writing.

You can submit a passage here to from your writing to determine the Fog Index and work out who your writing is likely to appeal to.

Note: you can even type in your website url and it will scan all the text in your website to give it a rating. For example, ThatActionGuy.com turns out to have a Fog Index rating of 10.56, which places it halfway between Time / Newsweek and The Wall Street Journal...which doesn't sound like a bad place to be to me!

Anyway, here's an article that explains everything you need to know.

Enjoy!

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Interpreting the Results of the Fog Index

This service analyses the readability of all rendered content. Unfortunately, this will include navigation items, and other short items of content that do not make up the part of the page that is intended to be the subject of the readability test. These items are likely to skew the results. The difference will be minimal in situations where the copy content is much larger than the navigation items, but documents with little content but lots of navigation items will return results that aren't correct.

Philip Chalmers of Benefit from IT provided the following typical Fog Index scores, to help ascertain the readability of documents.

Typical Fog Index Scores Fog Index Resources:

6 TV guides, The Bible, Mark Twain
8 Reader's Digest
8 - 10 Most popular novels
10 Time, Newsweek
11 Wall Street Journal
14 The Times, The Guardian
15 - 20 Academic papers
Over 20 Only government sites can get away with this, because you can't ignore them.
Over 30 The government is covering something up

Gunning-Fog Index

The following is the algorithm to determine the Gunning-Fog index.

*Calculate the average number of words you use per sentence.
*Calculate the percentage of difficult words in the sample (words with three or more syllables).
*Add the totals together, and multiply the sum by 0.4.
*Algorithm: (average_words_sentence + number_words_three_syllables_plus) * 0.4

The result is your Gunning-Fog index, which is a rough measure of how many years of schooling it would take someone to understand the content. The lower the number, the more understandable the content will be to your visitors. Results over seventeen are reported as seventeen, where seventeen is considered post-graduate level.

Flesch Reading Ease

The following is the algorithm to determine the Flesch Reading Ease.

*Calculate the average number of words you use per sentence.
*Calculate the average number of syllables per word.
*Multiply the average number of syllables per word multiplied by 84.6 and subtract it from the average number of words multiplied by 1.015.
*Subtract the result from 206.835.
*Algorithm: 206.835 - (1.015 * average_words_sentence) - (84.6 * average_syllables_word)

The result is an index number that rates the text on a 100-point scale. The higher the score, the easier it is to understand the document. Authors are encouraged to aim for a score of approximately 60 to 70.

Flesch-Kincaid grade level

The following is the algorithm to determine the Flesch-Kincaid grade level.

*Calculate the average number of words you use per sentence.
*Calculate the average number of syllables per word.
*Multiply the average number of words by 0.39 and add it to the average number of syllables per word multiplied by 11.8.
*Subtract 15.50 from the result.
*Algorithm: (0.39 * average_words_sentence) + (11.8 * average_syllables_word) - 15.9

The result is the Flesch-Kincaid grade level. Like the Gunning-Fog index, it is a rough measure of how many years of schooling it would take someone to understand the content. Negative results are reported as zero, and numbers over twelve are reported as twelve.

Reading Level Algorithms

Readability is the measure of how easy it is to read and comprehend a document. Readability tests were first developed in the 1920s in the United States. They are mathematical formulas, designed to determine the suitability of books for American students at a certain age, or grade level. Automating the process was intended to make it easier for tutors, librarians, and publishers to determine whether a book would be suitable for its intended audience. The formulas are based around the average words to a sentence, and the average syllables used per word. As such, they tend to reward short sentences made up of short words.

Being mathematically based, readability tests are unable to determine the likelihood that the document is comprehensible, interesting, or enjoyable. It's possible to obtain good readability scores with gobbledygook, providing the content contains short sentences made up of monosyllabic words. We'll leave the question as to why the word "monosyllabic" has five syllables for another day. Layout and design are also important factors to the readability of a document that cannot be determined using readability tests. Documents aimed at a higher level may require background knowledge, which cannot be determined by the tests.

For a document to be easily understood, the writing style should be clear and simple. This involves a writing style that is direct, and familiar to the intended reader. The structure of the document should be logical, unambiguous, and avoid redundant words.

Many of these factors cannot be measured using readability tests. Instead, readability tests provide a prediction of the reading ease for a document. Sentence length and polysyllabic words do have a direct impact on the readability of documents, albeit a surface measure of the characteristics of the text. They provide an indication that the content may be too dense with a quantifiable measure. The results should be used in conjunction with good writing style guidelines.

Guideline 14 of the Web Content Accessibility Guidelines requires that documents are clear and simple. Readability tests can provide a rough guide to the likelihood of a document being clearly understood. This service is to provide content authors with a guide to the readability of their website.

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Cheers!
Brian M Logan
ThatActionGuy.com
EMAIL ME HERE

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