|Click for larger view, source: Federal Reserve|
Here is how the commercial paper interest rate indexes are calculated.
"To calculate CP (commercial paper) interest rate indexes, the Federal Reserve Board uses DTCC's data for certain trades to estimate a relation between interest rates on the traded securities and their maturities. In this calculation, the trades represent sales of CP by dealers or direct issuers to investors (that is, the offer side) and are weighted according to the face value of the CP so that larger trades have a greater effect on the resulting index. With the relation between interest rates and maturities established, the reported interest rates represent the estimated interest rates for the specified maturities.
Interest rates calculated through the process described above are a statistical aggregation of numerous data reflecting many trades for different issuers, maturities, and so forth. Accordingly, the reported interest rates purport to reflect activity in certain segments of the market, but they may not equal interest rates for any specific trade. As with other statistical processes, this one is designed to minimize the difference between the interest rates at which actual trades occur and the estimated interest rates."