By G. Harry McLaughlin
My readability formula SMOG estimates the years of
education needed to fully understand a piece of writing. To use a free online tool which instantly calculates SMOG and four other
readability formulas click here.
SMOG is widely used,
particularly for checking health messages. Its estimates correlate closely with those yielded by the much more recent and accurate ATOS for Text.
Because SMOG uses the criterion of 100 percent comprehension it generally predicts levels at least two grades higher than Flesch-Kincaid and other popular formulas which have a comprehension criterion of 75%. [Do you want to fly in planes serviced by mechanics who have only a 75% comprehension of the maintainance manual?]
SMOG is easy to calculate using an approximate formula ó count the words
of 3 or more syllables in 3 samples of 10 sentences each, estimate the countís square root, and add 3.
A 2010 study in the Journal of The Royal College of Physicians of Edinburgh calls SMOG the gold standard and concludes that it should be the preferred measure of webpage readability.
However when SMOG was published in 1969 BC [Before Computers] it was intended only to provide readability estimates which could be easily calculated in one's head.
More reliance should therefore be placed upon ATOS for Text, a proprietary system derived from data on more than 3 million students reading 28,000 books.
The precise formula for SMOG yields an outstandingly high 0.985 correlation with the grades of readers who had 100% comprehension of test materials.
Here is the formula generalized for more than 30 sentences:
The standard error of the estimated grade level is 1.5159 grades, comparable
to that of other readability formulae. For a pdf
file of the original paper click G. Harry
McLaughlin (1969) SMOG grading: A new readability formula. Journal of Reading,
12 (8) 639-646.
You may have seen SMOG conversion tables compiled by one Harold C. McGraw.
They are slightly inaccurate because they are based on the approximate
formula. Furthermore tables for texts of fewer than 30
sentences are statistically invalid, because the formula was normed
on 30-sentence samples.
A sketch of how SMOG came to be devised was published in a Plain Language at Work
Newsletter. Decades ago I suggested how readability
formulas could be improved in Temptations
of the Flesch and Proposals
for British Readability Measures.