Gunning fog index
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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.[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. Texts that require a close-to-universal understanding generally
require an index of less than 8.
Calculating the Gunning fog index
The Gunning fog index can be calculated with the following
algorithm:[2]
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
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.[3] 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
recognized that readers perceive independent clauses as complete
thoughts. This changes the sentence index for some writers more than
others. In the 1980s, literature eliminated this step in calculations,
perhaps because this grammatical distinction was beyond the ability of
automation. A defense of the full system was made by Judith Bogart
of Pennsylvania University in 1985 [4]. A review of subsequent
literature shows the simplified formula is mostly advised, as illustrated
in this university guide.[5] Nevertheless, some continue to reminded us
that a series of simple, short sentences does not automatically make
reading easier, referenced in the Nov 2, 2006 entry guide to
readability [6]
In some works, such as Gibbon's The History of the Decline and Fall
of the Roman Empire, the difference between a Fog score using the
modern algorithm shown above and a hand-done score using the
original formula, would vary greatly. A simple test took a random
footnote from the text: (#51: Dion, vol.l. lxxix. p. 1363. Herodian, l. v.
p. 189.) and used an automated Gunning Fog site [7] first using their
sentence count, and then the count of sentences plus independent
clauses. The site first calculated an index of 19.2 using automation
and then calculated an index of 12.5 after including manually counted
independent clauses. This dropped the indexed reading difficulty from
post-graduate to secondary school.[8]
Example
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 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.