Keep sentences to 15 words on average but no longer than 25 words. Use bulleted lists to break up a longer sentence whenever possible. The words in each bulleted item can be counted as a separate sentence, but don't use more than 3 bulleted items.
Taken from GOV.UK:
Do not use long sentences. Check sentences with more than 25 words to see if you can split them to make them clearer.
People with some learning disabilities read letter for letter - they do not bounce around like other users. They also cannot fully understand a sentence if it’s too long.
People with moderate learning disabilities can understand sentences of 5 to 8 words without difficulty. By using common words we can help all users understand sentences of around 25 words.
Evidence:
The Oxford Guide to Plain English recommends 15–20 words per sentence. It also says: ‘…if you regularly exceed 40 words, you’ll certainly weary and deter your readers.’
Jyoti Sanyal, author of Indlish (the book for every English-speaking Indian), said: ‘Based on several studies, press associations in the USA have laid down a readability table. Their survey shows readers find sentences of 8 words or less very easy to read; 11 words, easy; 14 words fairly easy; 17 words standard; 21 words fairly difficult; 25 words difficult and 29 words or more, very difficult.’
Author Ann Wylie said: ‘When the average sentence length in a piece was fewer than 8 words long, readers understood 100% of the story. At 14 words, they could comprehend more than 90% of the information. But move up to 43-word sentences and comprehension dropped below 10 percent.’
(3 points above taken from Sarah's book: Content design.)
Way back in the 90s on a tech comm degree course, we were told that you should try and limit long sentences to 16 words (but slightly over is okay, if unavoidable) for usability. For good flow, it was suggested that one short sentence followed by a long and then a medium or short worked well (when you can manage it).
This would have all been backed up with some sort of research/insight, but it was 25 years ago, so I can only remember the recommendations, not the data it was based on. A lot of the info came from US/Canadian universities I think.
There's some results towards the bottom of this study that relate to sentence length. This was based on scientific writing.
study in language use
https://insidegovuk.blog.gov.uk/2014/08/04/sentence-length-why-25-words-is-our-limit/
Like this research! Thanks.
WCAG has some notes about reading levels and sentence length and mentions 25 words in English [https://www.w3.org/TR/UNDERSTANDING-WCAG20/meaning-supplements.html]. However, there is the additional worry about line length which is rather different but affects ereading.
Schneps, M. H., Thomson, J. M., Sonnert, G., Pomplun, M., Chen, C., & Heffner-Wong, A. (2013). Shorter lines facilitate reading in those who struggle. PloS one, 8(8), e71161.
[http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0071161] "Short lines eliminate crowded text to the left, reducing regression. The effects of attention modulation by the hand, and of increased letter spacing to reduce crowding, were also found to modulate the oculomotor dynamics in reading, but whether these factors resulted in benefits or costs depended on characteristics, such as visual attention span, that varied within our sample."
This extract suggests that we are right to focus on sentence length, but doesn't offer optimum sentence lengths. I've included the caveats around the impact of other factors, not least because these tend to get underplayed when you focus on just the words.
"Since the initial introduction of reading level assessment in the 1950s, numerous reasons have been suggested as causes of reading difficulty. Among these causes, two factors consistently predicted the difficulty of a text and impacted comprehension: vocabulary and sentence length. More recent reviews of readability factors supported and reinforced the use of word length/difficulty and sentence length formulas for making predictions about readability. Although unable to adjust for word derivations; slang, style, and syntax; previously existing contextual knowledge; and personal interest in the subject matter, readability formulas have become a widely accepted method of estimating the average comprehension of a text by an average reader. It is important to note that other factors such as document legibility, length, print size, use of graphics, primary language different than English, and cultural relevance are also critical factors influencing reader comprehension."
From: https://files.eric.ed.gov/fulltext/EJ851863.pdf
This is an extract from an article in the journal of the American Center for Plain Language, Clarity.
"Research shows that it’s not the length of the sentence that matters, but its syntax and structure. Clear syntax helps readers to parse the text more quickly and hold in working memory the words of the sentence in their appropriate groupings while they process the meaning of the sentence. In this way, clear syntax reduces the burden on working memory. Moreover, good syntactic cues can help readers recall what the text says."
The full article is here: https://www.researchgate.net/profile/Karen_Schriver/publication/285927928_The_research_basis_of_plain_language_techniques_Implications_for_establishing_standards/links/5664c50208ae192bbf90aa85/The-research-basis-of-plain-language-techniques-Implications-for-establishing-standards.pdf
Some of the research referred to - on syntax is here: https://www.hcde.washington.edu/files/people/docs/Isakson_Spyridakis_Sem_Syn.pdf
The Plain English Campaign's Crystal Mark advocates average sentence length of 15-20 words - http://www.plainenglish.co.uk/services/crystal-mark/7-the-crystal-mark-standard.html
The Clear English Standard, reference by the the Plain Language Commission, is also an average of 15-20 words. https://irp-cdn.multiscreensite.com/aaf9e928/files/uploaded/PLCstyleguide25July2012.pdf
The Plain Language Association International has a standard which recommends writers "keep sentences between 15-20 words on average, with individual sentences no longer than 30-35 words" http://plainlanguagenetwork.org/plain-language/what-is-plain-language/
I know search ranking isn't the point, but popular Wordpress SEO plug-in Yoast says it takes account of sentence length as part of optimising for readability. (Also, they quote someone called Sarah Richards on "opening up, not dumbing down"…)
"Sentence length is one of the core issues that makes a text hard to read. If your sentences are too long – over 20 words – people might lose track of your point. Often, readers have to jump back a few words to find out what you mean. This very tiring and inefficient. Try to keep the number of words in a sentence in check. Shorten your sentences. Aim for easy understanding, not a complex literary masterpiece."
https://yoast.com/yoast-seo-readability-analysis/
I couldn't see anything specific about sentence length in Google's own guidelines for quality rating: https://static.googleusercontent.com/media/www.google.com/en//insidesearch/howsearchworks/assets/searchqualityevaluatorguidelines.pdf
@clearerworld told me this on twitter.
Some ideas based on all the research I read during my Masters in Communications (focused on what makes written comms work or not), and since then:
- Around the 1940s-50s, people trying to create readability formulas mostly focused on 2 factors of difficulty: syntax and vocabulary. They considered many ways to determine syntax complexity, based on the number of propositions, idea density, etc. The problem was that all these required actually analysing each sentence, a specialised, time-consuming task. They finally chose a shortcut: as longer sentences are more likely to be complex than short ones, they settled for sentence length (number of words) as a proxy for sentence complexity. Sentence length is just a proxy.
- there’s no official cut-out between understandable and non-understandable sentences. (For ex., nothing like ‘up to 20 words is ok, but 21 or more is bad.’) We can pick a number for convenience, for guidelines sake, but can’t take it too seriously.
- Another confusing element: sentence-length guidelines are often worded as an average. That comes from readability formulas, which use the average length of words and sentences. Fans of formulas may say things like: ‘It’s okay to have 35-word sentences! The only thing that matters is the average length of sentences in the whole text.’ And that’s BS, of course. While the average is the only thing that matters to readability formulas, real readers actually read real sentences. And any long ones have better be structured well.
- Things that impact sentence difficulty and comprehension often have to do with the limitations of short-term memory: if I have to keep things in mind to ‘link’ them with other words later in the sentence, that’s hard work.
- Hard to understand (for example): very long subject followed by a short verb; back-loaded sentences (where you have to keep the beginning in mind); garden-path sentences; enumerations at the beginning or in the middle of a sentence.
- A great illustration to show that structure is more important than wordcount: Martin Luther King’s Letter from Birmingham jail. It includes a 300+ word sentence! Should be considered a complete horror, impossible to understand, right? And yet, it works.
http://clarity-international.net/journals/63.pdf