SMOG index

The SMOG grade is a measure of readability that estimates the years of education needed to understand a piece of writing. SMOG is the acronym derived from Simple Measure of Gobbledygook. It is widely used, particularly for checking health messages. The SMOG grade 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.

The formula for calculating the SMOG grade was developed by G. Harry McLaughlin as a more accurate and more easily calculated substitute for the Gunning fog index and published in 1969. 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.

A 2010 study published in the Journal of the Royal College of Physicians of Edinburgh stated that “SMOG should be the preferred measure of readability when evaluating consumer-oriented healthcare material.” The study found that “The Flesch-Kincaid formula significantly underestimated reading difficulty compared with the gold standard SMOG formula.”

Formulae

To calculate SMOG

1. Count a number of sentences (at least 30)
2. In those sentences, count the polysyllables (words of 3 or more syllables).
3. Calculate using

This version (sometimes called the SMOG Index) is more easily used for mental math:

1. Count the number of polysyllabic words in three samples of ten sentences each.
2. Take the square root of the nearest perfect square
3. Add 3