On this blog—and in my writing workshops—I’ve written about things that affect reading ease. I’ve focused on the average number of syllables per word, words per sentence, and sentences per paragraph. However, Harold Evans’ Do I Make Myself Clear? Why Writing Well Matters, introduced me to a seven-factor list from Robert Gunning, creator of the fog index.
Factors that affect reading ease
Some of the seven factors relate to length. They’re similar to the syllable, sentence, and paragraph measures. They also feed into the fog index. Evans describes the fog index as follows:
If you want to be clear, count the average number of words in your sentences, count the number of words of three syllables (the percentage of hard words), total the two, and multiply by 0.4. The lower ranking on the fog index, the easier the reading…
The seven factors include:
- Average sentence length in words
- Percentage of simple sentences
- Percentage of strong verb forms
- Proportion of familiar words
- Proportion of abstract words
- Percentage of personal references
- Percentage of long words
Why do the other factors matter? I like #3, the percentage of strong verbs, and #4, the proportion of familiar words, because they typically make the writer’s intent easier to grasp.
I’m puzzled by #6, percentage of personal references.
As I see it, personal references could cut both ways. Requiring detailed knowledge of your personal life will make your writing harder to understand. On the other hand, comprehension will improve when you use “you” and referring to things your readers care about.
The fog index isn’t infallible
Gunning’s seven factors can help you assess your content’s reader-friendliness. But they’re not infallible.
As Evans says,
Combine readability statistics with common sense. You can write illogical nonsense and get a good score of readability; the classic proof is that if you enter your sample from the last word to the first, you get the same score. Metaphor, analogy, and satire are unrecognized, wit unappreciated. The formulas have tin ears for the rhythm of sentence variety, for word choice, for the energy in the writing.
Test your reading ease online
You can run your text through an online version of the fog index.
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Note: Updated March 24, 2024.