The fog factor is the average number of polysyllabic words (excluding personal names) per sentence in a sample of 200 words. I came across this simple formula in Jyoti Sanyal’s Indlish. Here is the scoring system in Sanyal’s own words:
Clear writing has a fog factor of between 2 & 3.
Below 2 may be childishly simple.
Above 3 may be rather FOGGY!
Though the formula is not pegged to a grade level, it helps in arriving at a quick estimate of the readability of a text. However, a quicker estimate is possible. All we need to do is choose, instead of 200 words, an equivalent sample of 10 sentences. Now, fog factor = P/10, where P is the number of polysyllabic words (excluding personal names) in 10 sentences.
First let us calculate the fog factor of some passages in Indlish and then compare the scores with those of Harry McLaughlin’s SMOG (simple measure of gobbledygook). Martin Cutts’ foreword to Indlish has a fog factor of 1.7; Ravindra Kumar’s introduction, 3.2; and Sanyal’s first chapter titled ‘See-sawing To Plain English’, 1.6.
The SMOG grades for these texts are 10.14 and 12.79 and 9.92 respectively. (The SMOG Grade = 3 + square root of P30, the number of polysyllabic words in 30 sentences.) Since we have taken a sample of only 10 sentences, the number of polysyllabic words may be multiplied by 3 to get P30.
Interestingly, the SMOG grades and the fog factor scores agree on the order of difficulty of the texts. But there is a strong discord on the interpretation of the grade/score. For example, the SMOG grade suggests that a person with 10 years of schooling can understand Cutts’ passage but the fog factor’s scoring system tells us that it ‘may be childishly simple’.
I re-read the sample passages and found that the SMOG grades are a better measure of the texts than the fog factor. But the fog factor is so very simple that I wouldn’t want to jettison it. A better scoring system will make the fog factor a very useful readability tool.