Word length top predictor of readability, according to 90 years of research
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.
The longer the sentence, the lower reader comprehension, according to research by the American Press Institute. The research, based on studies of 410 newspapers, correlated the average number of words in a sentence with reader comprehension.
In fact, the API research showed an almost one-to-one negative correlation between sentence length and understanding.
One of top 2 predictors of readability
The American Press Institute isn’t the first organization to correlate sentence length with comprehension. For nearly 140 years, dozens of research teams have been studying how sentence length affects readability. Together, their body of knowledge shows:
Shorter sentences lead to easier reading.
Indeed, writes readability expert William H. DuBay in Unlocking Language (PDF), sentence length and word length have been proven in the lab — again and again — to be the two strongest predictors of reading ease.
Let’s take a look at some of those studies.
1. 1880s: Sentences getting shorter.
A professor of English Literature at the University of Nebraska was the first person to study sentence length.
The average sentence length has dropped from 50 words during Shakespeare’s day to about 20 words today.
In the 1880s, Lucius Adelno Sherman took the first statistical look at writing when he started calculating sentence length in historical literature. In his book, The Analytics of Literature (1893), he shared how sentences were growing shorter over time. In:
- Pre-Elizabethan times: Sentences averaged 50 words
- Elizabethan times: 45 words
- Victorian times: 29 words
- Sherman’s time: 23 words
Today, sentences average 20 words, reports DuBay— a 60% reduction from pre-Elizabethan days.
2. 1928: Long sentences reduce readability.
Mabel Vogel and Carleton Washburne of Winnetka, Illinois, were the first researchers to statistically correlate writing traits with readability.
Nearly half of the writing traits affecting readability centered on sentence length and complexity.
In 1928, they published 19 writing attributes that reduced readability. Nearly half of those characteristics centered on sentence length and complexity.
Sentence length itself — number of words per sentence — had a .453 correlation with readability. That is, as you add words, you reduce readability.
3. 1935: Sentence length tops 17 elements of readability.
How can you write for adults who can barely read? That’s what two Chicago academics — University of Chicago’s William S. Gray and St. Xavier College’s Bernice Leary — set out to learn in 1935.
The No. 1 predictor of readability, Gray and Leary found, was the average number of words per sentence.
After analyzing 48 passages of about 100 words each, and giving 800 adults a reading comprehension test, Gray and Leary produced their landmark book in reading research, What Makes a Book Readable, published in 1935.
Then Gray and Leary listed 17 elements of readability with correlations of 35% or higher in hierarchical order. 40% of those elements related to sentences — 29% to sentence length specifically.
In fact, Gray and Leary found, the No. 1 predictor of readability was the average number of words per sentence.
4. 1965: Sentence length drives reading ease.
In a 1965 study sponsored by the National Science Foundation, Edmund B. Coleman used cloze scores instead of multiple-choice tests to test readability.
Sentence length and word length top two predictors of readability.
Cloze tests delete selected words from a passage and ask participants to fill in the blanks, like this: Cloze tests delete selected ____ from a passage and ____ participants to fill in ____ blanks. They’re considered a more accurate way to measure a person’s understanding of a passage.
Indeed, Coleman found that he could achieve a virtually perfect reading grade level correlation by measuring just four elements:
- Sentence length. Lower is better.
- Percentage of prepositions . These linking words — on, under, against and during, for example — make sentences longer and more complex. The more prepositions, the harder copy is to read.
- Percentage of one-syllable words. Shorter words increase comprehension.
- Percentage of pronouns . Words like I, you, he, she and it can create confusion if it’s not clear to which subject the pronoun refers.
Note that half of these elements — the first two — center on sentence length.
5. 1976: Sentence characteristics affect readability.
In 1976, readability expert G. R. Klare reviewed 36 readability studies. He found, among other things, that five sentence characteristics affected comprehension.
No. 1: Long sentences and clauses reduce comprehension.
6. 1981: Short, simple sentences dramatically increase readability.
In 1981, researchers Thomas Duffy and Paula Kabance conducted four experiments to measure the effects of word and sentence length on comprehension.
Shortening and simplifying sentences and words made the message 6 grade levels easier to read.
The study used four versions of narrative or expository passages:
- The original version
- One with simplified vocabulary
- One with simplified and shortened sentences
- One with simplified vocabulary and simplified, shortened sentences
Shortening and simplifying both sentences and words reduced the reading grade level from 11th to 5th, making the revised copy six grade levels easier to read.
Why are long sentences so hard to read?
Here’s the problem with long sentences: The subject, verb and object are too far away from each other. Your readers have to fly to JFK to pick up the subject, Uber to Chelsea to grab the verb and hightail it to the Upper East Side to find the object. Then they have to restring these elements together to figure out who’s doing what to whom.
How many times are they going to do that?!
The ROI on simplifying sentences is enormous: When you make your sentences shorter and easier to read, you increase the chances that readers will understand your message.
So get to it.
Sources: William H. DuBay, Unlocking Language (PDF), Impact Information (Costa Mesa, Calif.), 2006
DuBay, Working with Plain Language (PDF), Impact Information (Costa Mesa, Calif.), 2008
DuBay, Smart Language (PDF), Impact Information (Costa Mesa, Calif.), 2007
DuBay, The Principles of Readability (PDF), Impact Information (Costa Mesa, Calif.), 2004