The SMOG grade is a measure of readability that estimates the years of education needed to understand a piece of writing.
[3] 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.”[4] Applying SMOG to other languages lacks statistical validity.
Furthermore, tables for texts of fewer than 30 sentences are statistically invalid, because the formula was normed on 30-sentence samples.