What is the perfect sentence length?

James Scott
6 min readApr 8, 2019

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One of the first things I was taught as a journalist was to write short sentences. My editor said to imagine you’re talking to someone in a bar; you want to use short sentences and straightforward language if you’re going to keep their attention.

Six years later when I became a technical writer, I was given pretty much the same advice: be short and concise in your instructions, don’t include unnecessary words. It got me thinking, is there an ideal sentence length to aim for? And if so, how long is it?

The goldfish attention span myth

I’m going to try and keep this next paragraph short, according some statistics circulated in the press, most of you won’t even finish reading this sentence. A Time magazine article claimed the average person loses concentration after about eight seconds while the average attention span of a goldfish was nine seconds.

A Time Magazine article claimed we have shorter attention spans than goldfish.

These claims would fit with the narrative we are becoming a race of smartphone-addicted zombies who are continually flitting from one app to the next. However, the goldfish myth was later busted by the BBC, who found it was a complete fabrication. There was no scientific evidence to suggest that our attention spans are getting shorter and ironically, in defence of the goldfish, they are actually used for studying memory formation.

Our attention span is like a strobe light

So what are facts? What do studies say about our attention spans? Scientists from Princeton and Berkeley discovered that our attention span actually pulses in and out up to four times every second — much like a strobe light.

This fluid attention span gives us the opportunity to switch attention to something else. Researcher Ian Fiebelkorn said:

“Attention is fluid, and you want it to be fluid. You don’t want to be over-locked on anything. It seems like it’s an evolutionary advantage to have these windows of opportunity where you’re checking in with your environment.”

Do we scan rather than read?

Perhaps linked to humans’ fluid attention span, most modern readers have also been found to scan text rather than reading every word.

Photo by Romain Vignes on Unsplash

In a reading and comprehension study, John Morkes and Jakob Nielsen challenged a group of test subjects to learn as much as possible from reading website text written in five different writing styles (a control, concise, scannable, objective and combined). They found people who read the concisely written text scored 58% higher than those who read the other writing styles. The paper concluded:

“Our study suggests that scannable, concise, and objective writing styles each make a positive difference in Web users’ performance and subjective satisfaction […] the combined effect of employing all three improvements was much larger than any of the individual improvements taken alone: our combined version recorded a 124% improvement in measured usability.”

An argument for shorter sentences

While researching for a past post, I came across some research by the American Press Institute presenting evidence there was a correlation between shorter sentences and reader comprehension. They found:

  • For sentences that were 8 words or less, readers understood 100% of the information.
  • For sentences that were 14 words long, readers understood 90%.
  • For sentences that were 43 words or longer, comprehension dropped to less than 10%.

There has also been a noticeable shortening of the sentence lengths in popular fiction. Andrew Moore, the editor of science journal BioEssays, noted how the mean sentence length of JK Rowling’s books were much shorter than that works of Dickens and Chaucer for example.

He found the mean sentence lengths were 49 for Chaucer, 20 for Dickens and 12 for Rowling. While he notes they aren’t strictly comparable given some of Rowling’s work is aimed primarily at children but he said it reflected a trend:

“Both Rowling and Dickens appeal(ed) to a broad public, and literary analysts agree on the general trend in MSL — a trend that is accelerated by computers and the Internet. My own analysis has a simple message: full length science papers need substantially shorter sentences if they are to be thoroughly read by most of their potential readership.”

Should you vary sentence lengths?

Author Mike Markel suggests that an average of 15 to 20 words is the most effective for technical communication. However he also advised against writing sentences that were all the same length:

“Sometimes sentence length affects the quality of the writing. In general, an average of 15 to 20 words is effective for most technical communication. A series of 10-word sentences would be choppy. A series of 35-word sentences would probably be too demanding. And a succession of sentences of approximately the same length would be monotonous.”

I think he hits the nail on the head here. While there seems to be a valid argument for writing concise sentences that you can scan quickly, writing too many short sentences becomes too dull and repetitive. The late writing instructor Gary Provost, wrote a wonderful passage to illustrate this point:

“This sentence has five words. Here are five more words. Five-word sentences are fine. But several together become monotonous. Listen to what is happening. The writing is getting boring. The sound of it drones. It’s like a stuck record. The ear demands some variety. Now listen. I vary the sentence length, and I create music. Music. The writing sings. It has a pleasant rhythm, a lilt, a harmony.

“Don’t just write words. Write music” — Gary Provost

I use short sentences. And I use sentences of medium length. And sometimes, when I am certain the reader is rested, I will engage him with a sentence of considerable length, a sentence that burns with energy and builds with all the impetus of a crescendo, the roll of the drums, the crash of the cymbals — sounds that say listen to this, it is important. So write with a combination of short, medium, and long sentences. Create a sound that pleases the reader’s ear. Don’t just write words. Write music.”

Resources

  1. You now have a shorter attention span than a goldfish — Time Magazine, May 15, 2015 http://time.com/3858309/attention-spans-goldfish/
  2. The spotlight of attention more like a strobe — https://www.princeton.edu/news/2018/08/22/spotlight-attention-more-strobe-say-researchers
  3. “Concise, Scannable, and Objective: How to Write for the Web by John Morkes and Jakob Nielsen (1997) Abstract Studies — http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.558.3130&rep=rep1&type=pdf
  4. The long sentence: A disservice to science in the Internet age — Andrew Moore https://onlinelibrary.wiley.com/doi/pdf/10.1002/bies.201190063
  5. Technical Communication, Mike Markel — https://michaltutoring.files.wordpress.com/2018/02/technical-communication-11th-edition-by-mike-markel.pdf
  6. Technical Communication, 9th ed. Bedford/St Martin’s, 2010 — Mike Markel.
  7. 100 Ways to Improve Your Writing — Gary Provost

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James Scott

Technical writer. I write about technical writing, documentation tools & trends, API documentation, AI and the etymology of words used in technology.