The funded status deficit of the 100 largest corporate defined benefit pension plans increased by $15 billion during April as measured by the Milliman 100 Pension Funding Index (PFI). The $258 billion deficit at the end of April is primarily due to a drop in the benchmark corporate bond interest rates used to value pension liabilities. Asset improvements helped to partially offset the full extent of liability increases in April. As of April 30, the funded ratio fell to 84.7%, down from 85.3% at the end of March. This April 30 PFI publication reflects updated asset returns for the first quarter of 2014.