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I am confused about whether it should be

A machine breaks down - on average - three times per day.

or

A machine breaks down on average, three times per day.

Miz
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    How about two commas? – Isabel Archer May 17 '20 at 10:48
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    Maybe: "On average, a machine breaks down three times per day." Or "A machine breaks down three times per day on average"? – Jules Cocovin May 17 '20 at 10:53
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    ... Yes, I prefer @Jules's way of writing it, but your way would be common in speech. As Isabel implies, << on average >> is essentially a parenthetical, needing the same setting off at each end. I'd say dashes (not hyphens) are too heavy-duty here, and brackets would only be appropriate if there were some reason to emphasise the averaging involved (unlikely). Commas are lightweight enough, indicating short pauses in speech. I'd choose zero punctuation at each end here as I'd leave no pauses in enunciating – this is a rare example where zero punctuation around a parenthetical works. – Edwin Ashworth May 17 '20 at 11:25
  • Does this answer your question? Usage of brackets/parentheses – Edwin Ashworth May 17 '20 at 11:31
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    No commas are required. "On average" and "three times per day" are sufficiently integrated into the clause as to not require their boundaries to be marked by punctuation. – BillJ May 17 '20 at 14:55

1 Answers1

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No punctuation is necessary. You can swap in a different adverb to see:

A machine breaks down about three times per day.

Here are some more examples of on average in action:

Corpus of Contemporary American English: on average

If you want to make on average nonessential—an aside—then use punctuation (commas, em dashes, parentheses) on both sides of it:

A machine breaks down, on average, three times per day.

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