20 CFR 225.41: How a cost-of-living increase is determined and applied.

Depending on the condition of the social security trust funds, the increase can be based on rises in either the consumer price index as published by the Department of Labor or the average wage index which is the average of the annual total wages used for computing a PIA. The increase is payable when the appropriate index for the third calendar quarter of one year shows an increase of at least three percent over the same index for the third calendar quarter of the previous year (or the last calendar quarter within which a legislated general benefit increase became effective). No increase is payable for the calendar year that immediately follows a year in which a legislated general benefit increase was effective. The increase amount is determined by multiplying the PIA by the percentage increase in the appropriate quarter of a previous year.