Second quarter levels are benchmarked annually to U.S. Census Bureau, Annual Survey of Public Pensions: State & Local Data, Table 1: National Summary of Public-Employee Retirement Systems, Corporate Bonds. For quarters more recent than the benchmark, growth rates are estimated using U.S. Census Bureau, Quarterly Survey of Public Pensions. Series is converted to quarterly by interpolation. Market value level is converted to book value using the Merrill Lynch U.S. Corporate Master par weighted price index. Unadjusted flow is the change in the level; data for the most recent ten years show no significant seasonality.