SMOG Simple Measure of Gobbledygook

By G. Harry McLaughlin

My readability formula SMOG estimates the years of education needed to understand a piece of writing. SMOG is widely used, particularly for checking health messages. 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.

But ATOS for Text has been much better validated than any other readability formula. It uses a dictionary giving the difficulty of each word (determined from 650 normed reading texts, a corpus of 474 million words, and the reading records of 30,000 students tested on 950,000 books). ATOS for Text is proprietary, but a free calculator is online. If you copy text into this Text Analyzer it will return an ATOS level together with a table showing the range of educational grades attained by people who generally read at various levels.

If you still want to use SMOG use click here for a free online tool which calculates it and three other readability measures.

SMOG was published in 1969 BC [Before Computers] so I made calculating a textís readability easy by offering an approximate formula ó count the words of 3 or more syllables in 3 10-sentence samples, estimate the countís square root, and add 3.

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 I offered for ease of calculation. Tables for texts of fewer than 30 sentences are statistically invalid as well, 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 readbility formulas could be improved in Temptations of the Flesch and Proposals for British Readability Measures.

Copyright (c) 2008 G. Harry McLaughlin. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.