§ 591.218 How does OPM compute price indexes?
Except for shelter and energy utilities, OPM averages by item the prices collected in each survey area. For the Washington, DC, area, OPM computes a simple average for each item based on the average prices from each DC survey area. On an item-by-item basis, OPM divides the COLA survey area average price by the DC average price and produces a price index.