"To compensate for the effects of inflation, Social Security recipients usually receive an annual cost-of-living adjustment (COLA). Benefits will be increased by 1.7% in 2015, following an increase of 1.5% in 2014. Social Security COLAs are based on changes in the Consumer Price Index for Urban Wage Earners and Clerical Workers (CPI-W), updated monthly by the Department of Labor's Bureau of Labor Statistics (BLS). The COLA equals the growth, if any, in the index from the highest third calendar quarter average CPI-W recorded (most often, from the previous year) to the average CPI-W for the third calendar quarter of the current year. The COLA becomes effective in December of the current year and is payable in January of the following year. (Social Security payments always reflect the benefits due for the preceding month.) […] Because a COLA of 1.7% will be paid to Social Security beneficiaries in 2015, identical percentage increases in Supplemental Security Income (SSI) and railroad retirement 'tier 1' benefits will be paid, and other changes in the Social Security program will be triggered. Although COLAs under the federal Civil Service Retirement System (CSRS) and the federal military retirement program are not triggered directly by the Social Security COLA, these programs use the same measuring period and formula for computing their COLAs. As a result, their recipients similarly will receive a 1.7% COLA in January 2015."
CRS Report for Congress, 94-803