Grammatically Correct: The writer's essential guide to punctuation ...

STYLE
ensure randorrmess), and calculate the average sentence lengtr
(that is, 100 divided by the number of sentences). Then count th
number of words in the sample that have three or more syllable
Add these two numbers together, and multiply the sum by 0.4. Tht
resulting value gives you the minimum number of years of educatio1
required by a reader in order to easily follow your writing. Repeat
the calculation with several more samples, and average the results.
For example, consider the first hundred words in the preceding
paragraph (beginning with Objective determinations and ending
with three or more syllables). This chunk contains five sentences,
for an average of 20 words per sentence, and 14 words of three
syllables or more. Thus, its Fog Index is: (20 + 14) X 0.4 = 13.6.
This value works out to a high school education, or high school
plus one or two years of college or university, depending on the
educational system. The number is high enough that readers with
more education shouldn't feel that the text is talking down to them,
but those with less education might find the wording challenging to
follow. (Obviously, a single calculation is not likely to be representative
of an entire book, so this process would have to be repeated a
number of times to produce a meaningful number.)
In practice, the chunks you use for your calculations will probably
be slightly under or slightly over one hundred words, since you don't
want to end a sample in the middle of a sentence. To calculate the
index for a chunk of any length, count (1) the number of words in
the chunk, (2) the number of sentences and (3) the number of
polysyllabic words. Divide (1) by (2) to get the average number of
words per sentence. Divide (3) by (1) and multiply by 100 to get
the percentage of polysyllabic words. Then add these last two values
and multiply their sum by 0.4, as before.
The Fog Index has been around a long time, but is being supplanted
these days by computerized tools that are faster and more
sophisticated. The accuracy of online tools is greater, since they
can scan an entire document rather than relying on random sampling.
Many grammar-checkers today include this type of feature, so you
may want to look for it if you are shopping around for a word
processor program.
Keep in mind that this type of measure is rather simplistic, since
length is just one aspect of reading difficulty. Many sophisticated
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