What is the Fog Index and should I use it?

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What is the Fog Index and should I use it?
We’ve all seen an article or page so full of industry jargon that it was almost impossible to read. But how do you make sure that your pages aren’t difficult or confusing for your readers? You could use the Gunning Fog Index.

What is the Fog Index?

Two weeks ago, I wrote a blog about new words/phrases the Oxford English Dictionary added and the Fog Index was one of them. The Fog Index is a kind of test that allows you to see how difficult a passage is to read. Specifically, it indicates the reading grade level necessary to understand the text.

How does it work?

To get your text’s Fog Index, you take a 100-word passage and analyze it for sentence and word length. The formula is as follows:

Reading Level = (Average number of words in a sentence + Percentage of words with three or more syllables) x 0.4

So let’s take a passage from a book I’m currently reading (Wool by Hugh Howey):

“The treads, like his father’s boots, showed signs of wear. Paint clung to them in feeble chips, mostly in the corners and undersides, where they were safe. Traffic elsewhere on the staircase sent dust shivering off in small clouds. Holston could feel the vibrations in the railing, which was worn down to the gleaming metal. That always amazed him: how centuries of bare palms and shuffling feet could wear down solid steel. One molecule at a time, he supposed. Each life might wear away a single layer, even as the silo wore away that life. Each step was slightly bowed from the generations of traffic, the edge rounded down like a pouting lip.”

Note: This passage is 113 words long.

If the piece is long, the State University of New York College at Buffalo recommends taking several 100-word passages and averaging all scores.

So let’s analyze this passage. I’ve highlighted all words with three or more syllables.

Average number of words in a sentence = 113 words / 8 sentences = 14.125

Percentage of words with three or more syllables = 6 words with 3 or more syllables/113 words = 0.053

To get the percentage, we need to multiply .053 by 100.

0.053 x 100 = 5.30

(14.125 + 5.30) x 0.4 = 7.77

According to the Gunning Fox Index, this passage requires a grade level of 7.77 (7th grade).

Let’s consider another example from Freakonomics by Steven D. Levitt:

“When a woman does not want to have a child, she usually has good reason. She may be unmarried or in a bad marriage. She may consider herself too poor to raise a child. She may think her life is too unstable or unhappy, or she may think that her drinking or drug use will damage the baby’s health. She may believe that she is too young or hasn’t yet received enough education. She may want a child badly but in a few years, not now. For any of a hundred reasons, she may feel that she cannot provide a home environment that is conducive to raising a healthy and productive child.”

Average number of words in a sentence = 112 words / 7 sentences = 16

Percentage of words with three or more syllables = 9 words with 3 or more syllables / 112 words = 0.08

0.08 x 100 = 8

(16+8) x 0.4 = 9.6

This passage requires a 9.6 reading level (9th grade).

Why should I use it?

The Fog Index has its shortcomings. For example, you may receive a higher or lower score depending on the passage you choose. Some passages are just more complicated than others.

You might also receive a different score based on the amount of syllables in the words, rather than the actual complexity of the word. For example: silo in the first passage might be a more difficult word for someone to understand than centuries, but the Index does not count two-syllable words.

Even with its limitations, it can help you figure out if the text on your site is readable. For example, if you analyze a passage on your site and find that it has a Fog Index of 17, your readers would likely need to read at a college level to understand your passage.

For your passage to be easily understood by almost everyone, you should aim for a score of 7 or 8.

If you want to check it against another formula, you can use the Flesch formula. The Fog Index is similar to, but a bit less complicated than, the Flesch Reading Ease Readability Formula used by Yoast SEO.

Using the Flesch Formula

Flesch’s formula is:

Reading ease = 206.835 – 1.015(total words/total sentences) – 84.6(total syllables/total words)

The score is out of 100. 90-100 is very easy to read, 0-30 is difficult. Our first passage has a reading ease score of 83.36, which is a 6th grade reading level.

Our second passage has a 77.31 score, which is a 7th grade reading level.

How can I use the Fog Index to make my page more accessible?

Once you figure out your passage’s Fog Index, you can figure out if switching out a few words will make your text easier to read. For example, this combination of passages from Medscape about myocardial infarctions (i.e., heart attacks) has a Fog Index of 16.76 (college senior).

Myocardial infarction (i.e., heart attack) is the irreversible death (necrosis) of heart muscle secondary to prolonged lack of oxygen supply (ischemia). Approximately 1.5 million cases of myocardial infarction occur annually in the United States. See the images below. Acute myocardial infarction, reperfusion type. In this case, the infarct is diffusely hemorrhagic. There is a rupture track through the center of this posterior left ventricular transmural infarct. The mechanism of death was hemopericardium. Patients with typical myocardial infarction may have the following prodromal symptoms in the days preceding the event (although typical STEMI may occur suddenly, without warning).”

Now let’s just replace myocardial infarction with heart attack (and remove i.e., heart attack in the first sentence) and see how much lower the reading level is.

With that one change, the new reading level is 13.59, a full three grade levels lower. And there are several other words, e.g., infarct, prodromal, etc., we could further target to bring the reading level down.

It’s complicated and it takes some time to do, but your readers will thank you for it.

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