Sentence length

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.)

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