Per US media, the US economy grew 3.1% in 2018 Q4/Q4 and 2.9% in 2018 Y/Y. 2.9% seems the reference number in various databases (World Bank, Reuters, Bloomberg, etc) but what exactly does Q4/Q4 mean and which measures changes from Jan 1 2017 to Dec 31 2017 compared to Jan 1 2018 to Dec 31 2018
If you could include the source(s) you were talking about that would be great. Without further info, my initial guess is that the Q4/Q4 means you compare GDP in 2018Q4 to GDP in 2017Q4. Y/Y means you compare GDP in the whole year of 2018 to that of 2017.
Upon further inspection, if you take a look at the US's real GDP, you'd get the following:
- Growth in 2018Q4 compared to 2017Q4: 2.52%
- Growth in 2018Q3 compared to 2017Q3: 3.13% [Link]
- Growth in 2018 compared to 2017: 2.93% [Link]
So I think your Y/Y number is right... but the number for "2018Q4/Q4" might have been growth in Q3?
It seems like the number reported was the initial estimate (which gives YoY growth of 2018Q4 to be 3.1% and 2018 to be 2.9%) in February 2019. The third estimate (which is what FRED uses) was released in December 2019 and yields YoY growth as calculated above.
As mentioned, if you can identify the sources used for the reported figures, that should solve the problem. Art is correct in terms of the the unit of time with Q4/Q4 comparing growth from the same quarter from the prior year, and that the Y/Y based on the according calendar year.
His calculations appear to be correct as well, and my guess is that it was either misreported as Q4 growth annualized rate or misinterpreted as Q3/Q3. I got the same numbers from the same source, and I also compared the actual volume in chained 2012 dollars as a rough check. The volume measurement is likely an inflation baseline that I didn't verify to be the case.