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The SMOG Readability Formula, a Simple Measure of Gobbledygook

G Harry McLaughlin created the SMOG Readability Formula in 1969 through an article, SMOG Grading – A New Readability Formula in the Journal of Reading. SMOG Readability Formula estimates the years of education a person needs to understand a piece of writing. McLaughlin created this formula as an improvement over other readability formulas. You may come across SMOG as an acronym for Simple Measure of Gobbledygook, but it’s widely believed the title is a nod to Robert Gunning’s FOG Index.

** ( Use our free SMOG Readability Calculator to grade your text using the SMOG formula).

McLaughlin started his career as a sub-editor of the Mirror newspaper in London, but spent much of his life in Applied Psychology. He left the newspaper to pursue a doctorate in psycholinguistics at the University of London. He wrote a thesis titled, “What Makes Prose Understandable.” After teaching human communications at City University of London, he moved to Toronto, Canada where McLaughlin taught briefly at York University and then to the University of Syracuse, where he published his SMOG Formula in 1969.

The SMOG Readability Formula

Step 1: Take the entire text to be assessed.

Step 2: Count 10 sentences in a row near the beginning, 10 in the middle, and 10 in the end for a total of 30 sentences.

Step 3: Count every word with three or more syllables in each group of sentences, even if the same word appears more than once.

Step 4: Calculate the square root of the number arrived at in Step 3 and round it off to nearest 10.

Step 4: Add 3 to the figure arrived at in Step 4 to know the SMOG Grade, i.e., the reading grade that a person must have reached if he is to understand fully the text assessed.

SMOG grade = 3 + Square Root of Polysyllable Count

The SMOG Formula is considered appropriate for secondary age (4th grade to college level) readers.

The premises of McLaughlin’s SMOG Formula are:

1. A sentence is defined as a string of words punctuated with a period, an exclamation mark, or a question mark.

2. Consider long sentences with a semi-colon as two sentences.

3. Words with hyphen are considered as a single word.

4. Proper nouns, if polysyllabic should be counted.

5. Numbers that are written should be counted. If written in numeric form, they should be pronounced to determine if they are polysyllabic.

6. Abbreviations should be read as though unabbreviated to determine if they are polysyllabic. However, abbreviations should be avoided unless commonly known.

7. If the text being graded is shorter than 30 sentences, follow the steps below:

i. Count all the polysyllabic words in the text

ii. Count the number of sentences in the text.

iii. Divide the figures obtained in i. by the figure obtained in ii. to arrive at Average Polysyllabic Words per sentence.

iv. Multiply the figure obtained in iii. with the average number of sentences short of 30.

v. Add the figure obtained in iv. to the total number of polysyllabic words.

vi. Compare the number of polysyllabic words in the SMOG Conversion Table.

SMOG Conversion Table
 Total Polysyllabic Word Count  Approximate Grade Level (+1.5 Grades)
 1 - 6  5
 7 - 12  6
 13 - 20  7
 21 - 30  8
 31 - 42  9
 43 - 56  10
 57 - 72  11
 73 - 90  12
 91 - 110  13
 111 - 132  14
 133 - 156  15
 157 - 182  16
 183 - 210  17
 211 - 240  18

McLaughlin validated his formula against the McCall-Crabbs passages. He used a 100% correct-score criterion, whereas most formulas test for around 50%-75% comprehension. His formula generally predicts scores at least two grades higher than the Dale-Chall formula.

** ( Use our free SMOG Readability Calculator to grade your text using the SMOG formula).
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