Cost-of-Living Adjustments for Federal Civil Service Annuities [Updated October 10, 2019] [open pdf - 859KB]
From the Summary: "Cost-of-living adjustments (COLAs) for the Civil Service Retirement System (CSRS) and the Federal Employees Retirement System (FERS) are based on the rate of inflation as measured by the Consumer Price Index for Urban Wage Earners and Clerical Workers (CPI-W). COLAs for both CSRS and FERS are determined by the average monthly CPI-W during the third quarter (July to September) of the current calendar year and the third quarter of the base year, which is the last previous year in which a COLA was applied. The 'effective date' for COLAs is December, but they first appear in the benefits issued during the following January."
CRS Report for Congress, 94-834
Congressional Research Service: https://crsreports.congress.gov/