Idiosyncratic Variation of Treasury Bill Yields

I document a dramatic increase in the importance of two types of variation in Treasury bill yields beginning in the early 1980s. The first is idiosyncratic variation in individual short‐maturity (less than three months) bill yields. The second is a common component in Treasury bill yields that is not shared by yields on other instruments, such as short‐maturity privately‐issued instruments or longer‐maturity Treasury notes and bonds. Some evidence suggests the first type reflects increased market segmentation. These results have important implications for the calibration and testing of no‐arbitrage term structure models and interpreting tests of the expectations hypothesis.