NISA Investment Advisors' Pension Surplus Risk index fell 40 basis points in March to 9.4%.
As was the case in February when the index dropped 50 basis points, the March decline was also due to decreases in both the volatility of return-seeking assets and interest rates, a NISA news release said.
Annualized volatility for the S&P 500 was 10.4% in March, compared with 12.7% in February.
The index aims to be a forward-looking estimate of the funded status volatility of U.S. defined benefit pension funds, based on averages of the 100 largest corporate plans.
The news release said the average funded status of plans in NISA's universe remained unchanged at 89.3% in March.