Frequency aggregation converts higher-frequency data series into lower-frequency data series (e.g., from monthly data to annual data). In FRED, the highest data frequency is daily and the lowest data is annual. There are three aggregation methods available:

Average: This method calculates an average of the original series values and converts it to a lower frequency (e.g., for a monthly data series for 2012, the sum of the 12 monthly values are divided by 12 to produce an annual data value for 2012).

Sum: Similarly, this method adds data values to convert to a lower frequency (e.g., for a monthly data series for 2012, the 12 monthly value are added and the sum becomes the annual data value for 2012).

End-of-period: This method takes the last value at the end of the period and uses it to convert to a lower frequency (e.g., for a monthly data series for 2012, the December 2012 value would be used as the annual data value for 2012).