SMOG

SMOG (Simple Measure of Gobbledygook) is a readability formula that estimates the years of education needed to completely understand a piece of writing. SMOG is widely used, particularly for checking health messages. The SMOG formula yields a 0.985 correlation with a standard error of 1.5159 grades with the grades of readers who had 100% comprehension of test materials.

SMOG was published by G. Harry McLaughlin in 1969 as a more accurate and more easily calculated substitute for the Gunning-Fog Index. To make calculating a text's readability as simple as possible an approximate formula was also given — count the words of three or more syllables in three 10-sentence samples, estimate the count's square root (from the nearest perfect square), and add 3.

Applying SMOG to other languages lacks statistical validity.

Formulas
To calculate SMOG \mbox{grade} = 1.0430 \sqrt{ 30\times\frac{\mbox{number of polysyllables}}{\mbox{number of sentences}} } + 3.1291 $$
 * 1) Count a number of sentences (at least: 10 from the start of a text, 10 from the middle, and 10 from the end).
 * 2) In those sentences, count the polysyllables (words of 3 or more syllables).
 * 3) Calculate using

A version is also given which is more easily used for mental math and is sometimes known as the SMOG Index:
 * 1) Count the number of polysyllabic words, excluding proper nouns, in a sample of thirty sentences.
 * 2) Take the square root of the nearest perfect square
 * 3) Add 3