Sentence length

Recommendations

Keep sentences to 15 words on average and not longer than 25 words.

Split up longer sentences to make them clearer: create more than 1 sentence or use bulleted lists.

Usability evidence

GOV.UK:
"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."
https://www.gov.uk/guidance/content-design/writing-for-gov-uk

'Content design' by Sarah Richards, 2017, includes the following references:

  • 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' 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."

'The role of word difficulty and sentence length in text comprehension', 1982
https://apps.dtic.mil/dtic/tr/fulltext/u2/a114935.pdf

'The Influence of Semantics and Syntax on What Readers Remember', 1999
https://www.hcde.washington.edu/files/people/docs/Isakson_Spyridakis_Sem_Syn.pdf

WCAG Reading Level, 2008
https://www.w3.org/TR/UNDERSTANDING-WCAG20/meaning-supplements.html

'Readability Assessment of Internet-Based Consumer Health Information', 2008
http://rc.rcjournal.com/content/53/10/1310/tab-pdf

'The research basis of plain language techniques: Implications for establishing standards', 2008 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

'Readability Levels of Health-Based Websites: From Content to Comprehension', 2009
https://files.eric.ed.gov/fulltext/EJ851863.pdf

Clarity Journal no. 63 'Writing smaller', 2010
http://clarity-international.net/journals/63.pdf

Plain Language Commission style guide, 2012
https://irp-cdn.multiscreensite.com/aaf9e928/files/uploaded/PLCstyleguide25July2012.pdf

'Shorter Lines Facilitate Reading in Those Who Struggle', 2013
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0071161

'Sentence length: why 25 words is our limit', 2014
https://insidegovuk.blog.gov.uk/2014/08/04/sentence-length-why-25-words-is-our-limit/

'Towards a better measure of readability: Explanation of empirical performance results', 2015 https://www.tandfonline.com/doi/pdf/10.1080/00437956.1989.11435805

'What is plain language?', 2015
https://plainlanguagenetwork.org/plain-language/what-is-plain-language/

Text complexity, ATOS, and Lexile® Measures, 2016
https://www.renaissance.com/products/practice/accelerated-reader-360/atos-and-text-complexity/

'Understanding Plain English summaries. A comparison of two approaches to improve the quality of Plain English summaries in research reports.' 2017
https://researchinvolvement.biomedcentral.com/articles/10.1186/s40900-017-0064-0

'How to use Yoast SEO: The readability analysis?' 2018
https://yoast.com/yoast-seo-readability-analysis/

The Crystal Mark standard. No date
http://www.plainenglish.co.uk/services/crystal-mark/7-the-crystal-mark-standard.html

Related wiki pages

Sentence structure
Plain English
Jargon

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