What is the difference between the following two sentences?

She goes to the forest three times per week.

She goes to the forest three times a week.


A in that sense is a preposition meaning the same thing as per. Any dictionary will have that as a definition for a (Dictionary.com):

each; every; per: ten cents a sheet; three times a day.

Oxford Dictionary Online and Merriam-Webster have similar entries.

So there is no difference in meaning between your examples. As to usage differences, per is characterized as being more formal and careful, so you would see it more in academic or legal writing or precise instructions, while a would be more common in speech and normal writing.

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  • OED says " b. With adverbs of repetition (as once, twice, many times, oft a day); now classified as the indefinite article". So perhaps we can say that historically this usage of a was as a preposition, but that it is increasingly being analyzed as a determiner. – Mark Beadles Mar 22 '12 at 20:59
  • @Brett, Mark: My (slightly out-of-date) OED says under "per prep," Section III 2 a In distributive sense, following words of number or quantity in expressions denoting rate or proportion: For each…, for every…: = a prep.1 8 b, by prep. 24 c. See also per cent, cent1 2. – FumbleFingers Mar 22 '12 at 21:08
  • There is an archaic preposition that is now is chiefly a member of compounds (e.g., abroad, abreast, adrift, afloat, ashore, etc.). As I've commented elsewhere, you can replace the determiner a with a variety of other determiners, and there would be no motivation to say that they are prepositions too. – Brett Reynolds Mar 22 '12 at 21:08
  • @FumbleFingers: The OED says, "b. With adverbs of repetition (as once, twice, many times, oft a day); now classified as the indefinite article: see a adj. 4". – Brett Reynolds Mar 22 '12 at 21:09
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    @Brett Reynolds: Yes, I think the change was probably sensible. But like I said, usage has shifted anyway. And my OED calls each and every for example, "quasi-pronouns". In some contexts they're interchangeable with per, sometimes only with each other, and sometimes not even that. "Part of speech" categories are something of a blunt instrument when you get down to the details. – FumbleFingers Mar 23 '12 at 2:16

Strictly speaking, "per" and "a" are equivalent in OP's examples (effectively, meaning "in each", or "in every"), but it's worth pointing out that using "per" like this has overtones of science/academia, business/commerce jargon, legalese, etc., often implying a "commitment to accuracy".

Thus we wouldn't be surprised to discover that she (Little Red Riding Hood?) usually goes to the forest three times a week as a casual activity of her own choice (to pick flowers or whatever), where it's quite possible she actually goes twice or four times some weeks.

But if we're told she goes three times per week, that suggests a more formal arrangement. Perhaps because Granny will be taken into care if the family can't keep up a regular schedule of home visits to make sure she gets enough cake to eat, for example.

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They are both right, but a is more common.

per (preposition)

: with respect to every member of a specified group : for each

a (preposition)

: in, to, or for each

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    More common, maybe. But try writing in your physics paper "25 centimetres a second". – GEdgar Mar 22 '12 at 21:31

In general usage, the first feels more precise than the second. The former implies each and every week she goes to the forest exactly three times. The latter has a more general feeling - she goes to the forest about three times of the week, depending on the weather and her moods. As always, context is probably more important than the exact grammar in determining the meaning.

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There is a grammatical difference, in that per is a preposition followed by a noun phrase without a determiner. In the second sentence, a week is a typical noun phrase. Despite these grammatical differences, the standard interpretation of each sentence would be the same.

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    Believe it or not, "a" is usually regarded as a preposition in those circumstances, and that particular "a" arose from the preposition "on". – Mark Beadles Mar 22 '12 at 20:29
  • Nonsense! Wiktionary has this wrong. There is a preposition a, but it has absolutely nothing to do with this. It's a straight determiner, and can be replaced by other determiners (e.g., two times this week, two times each week, two times some weeks, etc.) – Brett Reynolds Mar 22 '12 at 20:43
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    Then the Online Etymological Dictionary has it wrong too. And so does the OED. Admittedly the OED does include a note for this sense, "now classified as the indefinite article", so it's open to some analysis. But it's certainly not "Nonsense!" – Mark Beadles Mar 22 '12 at 20:53
  • The OED says, "b. With adverbs of repetition (as once, twice, many times, oft a day); now classified as the indefinite article: see a adj. 4" because the previous analysis was clearly wrong. – Brett Reynolds Mar 22 '12 at 21:00
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    @MarkBeadles: I apologize for the tone of the reaction. – Brett Reynolds Mar 23 '12 at 0:20

When referring to people I would tend to use per, e.g. 24€ per person, or 24 kilos per passenger.

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Ditto Brett. I think the "a week" is simply wrong as the word "week" has no clear role in the sentence. I think people who say this are either contracting "per" to "a", or they are ommiting a preposition, like "three time IN a week".

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    See my comment to Brett - "a week" and similar forms are well-established in English, and came from abbreviation of the preposition "on". – Mark Beadles Mar 22 '12 at 20:30
  • It is perfectly normal for noun phrases to function as adjuncts (or what school grammar bizarrely likes to call "adverb phrases"). It happens all over the place: this week, I'm going; I was paid twice this week, etc. That's what's happening with a week. It's not wrong and it has nothing to do with prepositions. – Brett Reynolds Mar 22 '12 at 20:46
  • Christmas Comes But Once a Year. Or should that be "once in a year"? – FumbleFingers Jun 26 '12 at 2:22
  • @FumbleFingers Headlines and titles are not generally the best models for good grammar. – Jay Dec 27 '15 at 23:06
  • @BrettReynolds Hmm, but in "I'm going this week", "this week" acts as an adverb saying when you are going. But in "goes three times a week", I don't see any relationship established between "a week" and either "goes" or "three times". – Jay Dec 27 '15 at 23:09

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