The COFI is computed using several different factors, with interest paid on savings accounts comprising the largest weighting in the average. Because of this, the index tends to exhibit low volatility and follow market interest rate changes somewhat slowly; it is generally regarded as a two-month lagging indicator of market interest rates.
Because it is computed using data from three western states, the COFI is primarily used in the western U.S. while the Treasury Index is the measure of choice in the east.