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SMOG Index

SMOG is a readability formula that estimates the years of education needed to 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.

Numerous online calculators give the SMOG level of submitted text or a URL but only the SMOG Calculator listed below as an external link uses a dictionary to look up the syllable length of words. Because of this, the SMOG Calculator counts syllables more accurately than any other readability program.

In 2010 a 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-orientated healthcare material.” The study found that “The Flesch-Kincaid formula significantly underestimated reading difficulty compared with the gold standard SMOG formula.”

Applying SMOG to other languages lacks statistical validity.

Source: Wikipedia - SMOG