“The Fog Index is based upon counts of long words and of sentence length. It includes a factor to make it represent the approximate number of years of schooling needed to readily comprehend the prose being tested. Thus, the higher the Fog Index, the harder the writing is to read,” writes Robert Gunning in ‘How To Take The Fog Out Of Writing’.
According to the formula, FI = (ASL + P%) * 0.4. [FI — Fog Index; ASL — average sentence length; P — number of polysyllabic words in a sample of about 100 words.] From the list of polysyllabic words, Gunning omits capitalized words, easy compound words and disyllabic verbs made trisyllabic by adding -es or -ed. He also considers independent clauses as separate sentences, even if they are not separated by full stops. Once the FI is calculated, he ignores digits after the decimal point. A passage with a score higher than 17, is assigned an FI of 17+.
Three things have to be done:
1. ASL = W/S [W — number of words; S — number of complete sentences in passage]
2. P% = (P/W) * 100
3. Adding both and multiplying the result by 0.4.
The formula is simple enough. Does it have to be simplified? What harm if we can save a step or two without loss of reliability? Here we go!
FI = [(W/S) + (P/W) * 100] * 0.4
If we take a sample of 40 words, then
FI = [(40/S) + (P/40) * 100] * 0.4
On simplification, the Simplified Fog Index (SFI) = (16/S) + P.
Though Gunning suggested ‘a sample of at least 100 words’, he has tested the FI on smaller samples. He has tested two sentences of lengths 78 words and 90 words respectively and found they scored 17+. He has rewritten them to get an improved FI of 10. Let us test his revisions with the SFI though the passages contain more than 40 words.
Passage 1. “Typing errors are easy to make in transposing code numbers of appropriations. We suggest each Division Planning Office set up a file of yellow tickets showing all authorized unit and item numbers. Then each can make a daily check of construction charges before sending time distribution sheets to the Accounting Department.”
There are 3 sentences, 51 words and 5 polysyllabic words in the above passage. Note that FI does not count capitalised and simple polysyllabic words. FI = 10. And SFI = (16/3) + 5 = 10.33 = 10. In this case FI = SFI.
Passage 2. “Use of the new method for the finishing units suggests that settling is held back by minute amounts of emulsifier. These are very hard to remove. No matter how much care we take, present equipment still allows some of this substance, now and then, to be carried through the units or to be picked up from tank bottoms.”
There are 3 sentences, 58 words and 3 polysyllabic words in the above passage. FI = 10. And SFI = (16/3) + 5 = 8.33 = 8. In this case FI and SFI differ by two grades. SFI seems to be less accurate. This is my suggestion: Use the FI for grading books and the SFI for testing everyday texts.
Since SFI will be a rough estimate of readability, a few changes may be incorporated in its application. First, take a sample of complete sentences in about 50 words. Be severe and count all polysyllabic words. Then apply the SFI.