Sentence Length Variance

Sentence length variance measures how widely sentence lengths spread around their average in a passage, reported as variance or standard deviation in words. High variance signals varied rhythm; low variance signals uniform, metronomic prose.

The numbers

Recommended average and maximum sentence length for readable content: 15 words average, 25 words maximum (Readability Guidelines)
Oxford Guide to Plain English recommended sentence length, with a warning that regularly exceeding 40 words will weary readers: 15 to 20 words (Readability Guidelines)
Comprehension by average sentence length (Ann Wylie): under 8 words fully understood; 43-word sentences barely understood: under 8 words: 100%; 14 words: over 90%; 43 words: below 10% (Readability Guidelines)
US press associations' readability table, cited by Jyoti Sanyal: 8 words or less very easy; 11 easy; 14 fairly easy; 17 standard; 21 fairly difficult; 25 difficult; 29 or more very difficult (Readability Guidelines)
Sentence length people with moderate learning disabilities can understand without difficulty (GOV.UK), and the length common words make workable for all users: 5 to 8 words (around 25 with common words) (Readability Guidelines)
Typical average sentence length in popular science writing: about 20 words (ProWritingAid)

Definition

Sentence length variance measures how widely the sentence lengths in a passage spread around their average. Two passages can share the same mean length and read completely differently: one alternates short jabs with long, elaborate sweeps, while the other repeats near-identical lengths in a metronomic pattern. The average hides that difference; the variance exposes it.

The statistic is ordinary variance: take each sentence's word count, subtract the mean, square the deviations, and average them. Its square root, the standard deviation, is usually the more readable figure because it stays in the same unit, words per sentence.

Why spread matters as much as average

Most style guidance targets the average. The Readability Guidelines recommendation is to "keep sentences to 15 words on average and not longer than 25 words", and the Oxford Guide to Plain English recommends 15 to 20 words, warning that "if you regularly exceed 40 words, you'll certainly weary and deter your readers".

ProWritingAid points at the spread instead: "texts with lots of long sentences will exhaust your readers", while "texts with lots of short sentences will create a choppy, disjointed reading experience". A respectable average can conceal either failure. Variance is the number that catches both.

What the comprehension research shows

Sentence length maps directly onto comprehension. Writing researcher Ann Wylie, quoted in the Readability Guidelines: "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."

A readability table from US press associations, cited by Jyoti Sanyal, grades sentences of 8 words or less as very easy, 11 as easy, 14 as fairly easy, 17 as standard, 21 as fairly difficult, 25 as difficult, and 29 or more as very difficult. GOV.UK adds the accessibility case: people with moderate learning disabilities can understand sentences of 5 to 8 words without difficulty, and common words help all users understand sentences of around 25 words.

The link to burstiness and AI text

Sentence length variance is the practical core of burstiness. When tools report that AI text has a uniform rhythm, low sentence length variance is usually the concrete measurement behind the claim; QuillBot notes that generative AI "will often use more uniform, repetitive sentence structure".

Variance describes rhythm; it does not prove authorship. A human writing deliberately plain procedural prose can be as uniform as any model, and genre shifts the baseline: ProWritingAid puts popular science at about 20 words per sentence, with thriller and romance genres running shorter. Treat the number as a description, not a detector.

In the ScriptGrain analyser

Sentence length variation is one of the attributes ScriptGrain's free analyser at scriptgrain.com/tools/writing-style-analysis measures client-side. It splits your text into sentences, counts the words in each, and computes the spread with the arithmetic shown below, deterministic and fully inspectable.

Read it against intent. Uniformity on purpose, as in step-by-step instructions, is a choice; uniformity by accident is monotony. The variance figure tells you which of those conversations to have with your draft.

Worked example

A four-sentence paragraph: "The report was late. Nobody at the meeting mentioned it, which surprised her more than the delay itself. She checked the timestamp twice. Then she wrote a single line to the whole team and waited." Word counts per sentence: 4, 14, 5, 12.

Mean = (4 + 14 + 5 + 12) / 4 = 8.75 words. Deviations from the mean: -4.75, 5.25, -3.75, 3.25. Squared and summed: 22.5625 + 27.5625 + 14.0625 + 10.5625 = 74.75. Variance = 74.75 / 4 = 18.6875. Standard deviation = square root of 18.6875 = 4.32 words (to two decimal places).

An average of 8.75 words with a standard deviation of 4.32 means sentences typically land about four words above or below the mean: a varied, bursty rhythm despite the short average.

Sources

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