The expression is a compound adjective that, according to the OED, is “used with reference to a comparison of figures with corresponding ones for a date twelve months earlier.” The dictionary’s first citation is from a 1976 article in the Daily Telegraph: “It is hoped this will show a year on year rise in average earnings of between 14 and 15 per cent.”
The Google Ngram chart below showing use of year on year between 1975 and 2008 suggests it is both a Britishism (the blue line represents British use) and a NOOB (the red line, showing U.S. use, climbs steadily starting in the mid-1990s).
Certainly, it’s often been seen in U.S. financial pages in recent months, for example:
- “Consumer spending suffered its sharpest year-on-year drop since World War II, according to Italy’s leading business association.” New York Times, December 12, 2012
- “Home values in San Francisco have been growing on a year-on-year basis for four consecutive months.” San Francisco Chronicle, November 2012:
Interestingly, year on year has an (as far as I can tell) exact synonym: year over year. This one is an American speciality (to use a Britishism which has not yet appeared here). U.S. use of year over year is the green line in the chart, British use the yellow line. An example from the Associated Press, December 26, 2012: “October was the fifth straight month of year-over-year gains, after nearly two years of declines.” And the New York Times, December 28, 2012: “The pace of rental growth year over year has also slowed.”
The two expressions seem to be battling it out on these shores. I predict over will ultimately prevail. We like our literalisms even more than our Britishisms.