Because long ones reduce comprehension
Here’s the problem with long sentences: Every time you add a word, you reduce comprehension. Add another one, reduce it even further. Add another one, reduce it even further.
There’s almost a one-to-one correlation between sentence length and understanding, according to a study by the American Press Institute. The research, based on studies of 410 newspapers, found that with average sentences of:
- 8 words or less, readers understood 100% of the story.
- 14 words, readers understood 90% of the information.
- 43 words, readers understood less than 10%.
And that 107-word sentence your subject-matter expert made you write? After reading that sentence, your readers not only don’t know what they’ve read, they also forget where they parked the car. That’s a net loss of knowledge — not exactly our goal as communicators.
It’s not just the American Press Institute. Nearly 140 years of research shows that short sentences are easier to read and understand. In fact, sentence length has been proven in the lab — again and again — to be one of the top two predictors of reading ease. (Word length and familiarity is the No. 1 predictor.)
That’s because in long sentences, the subject, verb and object are too far away from each other. They have to rewrite the sentence in their heads so they can understand.
How many times are they going to do that? If it’s my sister’s oncologist, sure, I’ll do it all day long until I understand what’s going on. But if it’s a brand message — a news release or an email newsletter or blog post — I’m not going to work that hard, and neither are your readers.
Breathe or burn.
Because long sentences reduce understanding, one of my college professors used to make us read our long sentences aloud in front of the entire class. If we ran out of breath before we ran out of sentence, we got an embarrassing reminder of the importance of short sentences.
One professor out there is crueler than mine. He’d make his students light a match and read their sentences out loud. If they ran out of match before they ran out of sentence, they got a painful reminder of the importance of short sentences.
Why short sentences?
- Sentence length and complexity make up half of the 19 writing attributes that make messages harder to read, found researchers Mabel Vogel and Carleton Washburne in 1928. Those attributes included words per sentence, complex sentences and conjunctions, which link phrases together into longer sentences.
- Sentence length and the percentage of prepositions compose half of the four elements that achieve a virtually perfect reading grade level correlation, found Edmund B. Coleman in 1965. Prepositions combine clauses into longer sentences.
- Shortening and simplifying sentences made revised copy six grade levels easier to read, found researchers Thomas Duffy and Paula Kabance in 1981.
- Long sentences decrease comprehension, found readability researcher Ralph Ojemann in 1934. He found that sentences including prepositions and dependent clauses reduced understanding.
- The more indeterminate clauses a passage includes, the harder it is to understand, found Ralph Tyler and Edgar Dale in 1934. Why? Indeterminate clauses make sentences longer and increase the number of ideas per sentence.
- Five sentence characteristics — including length, passive voice and embedded clauses — affected comprehension, found readability expert G. R. Klare in 1976.
So … how long is too long?
 American Press Institute via Jon Ziomek, associate professor emeritus, Medill School of Journalism
 William H. DuBay, Unlocking Language (PDF), Impact Information (Costa Mesa, Calif.), 2006
 William H. DuBay, Unlocking Language: The Classic Readability Studies (PDF), Impact Information, 2006, p. 55
 William H. DuBay, Smart Language: Readers, Readability, and the Grading of Text (PDF), Impact Information, Jan. 25, 2007, p. 80
 William H. Dubay, The Principles of Readability (PDF), Impact Information, Aug. 25, 2004, pp. 39-40
 William H. Dubay, The Principles of Readability (PDF), p. 16
 William H. Dubay, The Principles of Readability (PDF), p. 39