The assumptions used by the Pension Benefit Guaranty Corp. last year to calculate its record-setting deficit were reasonable, an actuarial group says.

In 2012, the PBGC estimated that its single-employer insurance program was $29.1 billion in the red. That was the biggest deficit — the difference between the PBGC's assets and its liabilities — since Congress created the agency in 1974 as an integral part of the Employee Retirement Income Security Act.

The announcement of the deficit for the program, which is funded in part by premiums paid by employers with defined benefit plans and used to guarantee benefits to participants of failed plans taken over by the agency, prompted some employer benefits organizations to contend that the deficit was overstated due to the low interest rates used by the agency to calculate the deficit.

But the pension committee of the American Academy of Actuaries said Thursday in an issue brief that the methods and assumptions used by the agency produced a “reasonable representation of the PBGC's current obligation and deficit.”

%%break%%

The 3.28% interest rate used by the PBGC to estimate liabilities “stands at a reasonable level between the higher corporate bond yields used by private plans and lower Treasury securities rates considered to be 'risk-free,'” the Academy said in a statement.

PBGC officials welcomed that endorsement of its actuarial assumptions. “We're glad that a respected and independent group of experts has confirmed what we've been saying all along — the deficit is real and has been accurately stated for years,” PBGC Acting Chief Policy Leslie Krammerich said in an emailed statement.

The actuarial group acknowledged that while the PBGC does not face an immediate financial crisis, priority should be given to changing its premium structure so that the premiums paid by employers would be correlated with the risks their pension plans pose to the agency.

A premium structure correlated with the risk that plans pose to the system would “encourage sponsors to fund plans better and lessen the risk associated with potential terminations,” the Academy said in its eight-page report.

For several years, the PBGC has backed an approach in which the premiums employers would pay the agency would be based at least in part on their credit ratings.

In support of such a change, which would require congressional approval, PBGC Director Joshua Gotbaum said that pension plans sponsored by an employer with a top credit rating, for example, don't pose the same level of risk as those with a subpar rating.

Currently, employers pay a flat annual rate of $42 per plan participant, regardless of their financial strength or credit rating. In addition, employers with underfunded plans pay an additional premium of $9 per $1,000 of plan underfunding.