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If I want to show the comparison between rate in 2010 Jan and 2009 Jan, which of the following should I use?

  1. Comparison of rate between 2010 Jan and its prior month.
  2. Comparison of rate between 2010 Jan and its previous month.
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In my opinion, neither previous nor prior work in this context for what you want to express, because both expressions imply December 2009.

Also, 'between' is a bit vague, because while the context implies you are comparing the rates of the two months, 'between' carries the idea of comparing the rates for the whole period between those two months.

Why not simply 'Comparison of rates: Jan 2010 and Jan 2009'? It is not especially clever, but it is shorter than the two phrases you propose.

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Sorry. I totally misunderstood the question. I though the was talking about December 2009. – J D OConal Oct 11 '10 at 5:03
@JDOConal Don't apologize. You had the best answer. – Ariel Oct 15 '13 at 10:50

Neither "previous" nor "prior" are correct here, because they would refer to December 2009.

If you want to use a business analysis buzzword, you could use "year over year", as in

Year-over-year comparison of rates for January

If you are addressing normal people, however, I'd suggest simply

Comparison of rates between January 2010 and January 2009

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Shouldn't it be Comparison of rates between "January 2009" and "January 2010"? quotes are added to highlight – Tech Jerk Nov 11 '10 at 9:53
It depends on where your focus is. Both can be appropriate. Both are valid in any case. – Peter Eisentraut Nov 11 '10 at 19:57

The Oxford Corpus of English lists several example sentences for prior:

She visited me on the day prior to her death.

Mrs Scott had to spend two weeks in hospital, including a week prior to the Caesarian birth.

For previous:

The month previous to publication . . .

The starting grids are based on a reversal of the finishing order of the previous race.

Both prior and previous have similar definitions: 'existing or occurring before in time or order' and 'existing or coming before in time, order, or importance' respectively.

From these two points (examples of usage in English and dictionary definition), I can see no evidence that either word is more or less appropriate in your context. However, I would say the following sound better:

  • Comparison of rate between January 2010 and the prior month.
  • Comparison of rate between January 2010 and the previous month.

That is, I would use the rather than its, although your sentences are grammatical.

EDIT: Now that I understand that the OP was talking about comparing January 2009 and January 2010, I offer these suggestions:

  • Comparing the rates of January 2010 and the prior January.
  • Comparing the rates of January 2010 and the previous January.

I also agree that it would be easier, and perhaps better, just to say 'Comparing the rates of January 2009 and January 2010.*

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Thanks, then how can one differentiate between "2010 Jan vs 2010 Feb" and "2009 Jan vs 2010 Jan" comparison? – lokheart Oct 11 '10 at 4:08
I edited my post. I'm sorry -- I misunderstood your question. – J D OConal Oct 11 '10 at 5:04
+1 for everything after the edit – Ophiuroid Oct 11 '10 at 5:47

In general for comparing corresponding periods across years you use the expression year on year

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Neither prior nor previous are words necessary to convey the meaning of the statement. The clearest way, so that no one misunderstands the question, is to delineate it completely:

Comparison of rates between Jan 2009 and Jan 2010.

This "statement" implies that there is a list to follow with the said comparisons indicated. It is not meant to convey all of the actual comparisons. It seems to be more of a heading.

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