Plain English recommends short sentences. Robert Gunning faults marathon sentences in his book How To Take The Fog Out Of Writing. Though he admits to the possibility of long sentences being balanced and readable, he notes that only highly skilled writers such as Charles Dickens and Thomas Wolfe can write a marathon sentence with clarity. He adds: “But even these accomplished writers produced marathon sentences only occasionally. On the average, they wrote fewer than 20 words per sentence.”
Jyoti Sanyal’s Indlish has an interesting and informative chapter titled ‘Shrink or sink’ on the length of the sentence. He writes: “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.”
Martin Cutts, in his Oxford Guide To Plain English, offers the following guideline: “Over the whole document, make the average sentence length 15-20 words.” And what’s the reason? He explains: “More people fear snakes than full stops, so they recoil when a long sentence comes hissing across the page.”
Plain English also recommends short words. Even if the average sentence length of a document is 15-20 words, readability is not guaranteed. Polysyllabic words are likely to make the meaning of the document difficult to grasp. So we also need a guideline for average word length.
Words have two units of measure: syllables and letters. I examined the word list of Voice Of America and found that the average word length in syllables is 1.74 and that in letters is 5.67.
Now, we have been measuring sentences only in words. But sentences have three units of measure: words, syllables and characters. And so we may take the following as the new guideline: “Over the whole document, make the average sentence length 15-20 words, 25-33 syllables and 75-100 characters.”