Sponsors of collectively bargained multiemployer pension plans increased contributions to plans between 2009 and 2013.

Still, many plans saw their unfunded liability grow during that time, according to a new report by the Society of Actuaries.

The system’s aggregate contributions increases 6.9 percent a year during the period, more than triple the average inflation rate of 2.1 percent.

They also exceeded legally bound minimum required contributions. In 2009, aggregate contributions were 8.75 times required minimums, and 94 percent of all plans infused more cash than they were required.

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About 75 percent of plans were funded well enough to not have MRCs during the period.

Despite investing more than was required, many plans saw unfunded liabilities grow in 2013. Just how many depends on what discount rate is applied to factor future liabilities.

One rate, the plan actuary discount rate, is substantially higher than the so-called current liability discount rate, which is based off the yield on U.S. Treasuries.

The higher the discount rate, the lower future projected liabilities, which of course improves a plan’s funded status.

The average plan actuary discount rate during the period was 7.5 percent, much higher than current liability discount rate, which was as low as 3.74 percent in 2013.

The discount rate used by the Pension Benefit Guaranty Corp. to determine funded status zones under the Pension Protection Act of 2006 is higher than average plan actuary discount rate.

The three different discount rates create vast differences when calculating funded status. In 2013, when using PPA’s discount rate, aggregate unfunded liabilities were $115 billion. But when applying the current liability discount rate, unfunded liabilities swell to $500 billion.

In 2013, 99 percent of plans had an unfunded liability on a current liability basis, while there were 84 percent when applying PPA’s zone determination discount rate.

“Funded status results can vary dramatically under different measurement bases,” the report notes.

“There is no universally accepted view on the practical implications of the different bases,” according to the report.

That means a plan can appear to be well-funded when applying the actuary discount rate and poorly funded when applying the current liability rate, the report said.

The increase in unfunded liabilities during the period the SOA studied is also explained, in part, by a decrease in total active participants in multiemployer plans.

Between 2009 and 2013, total active participants declined from 3.9 million to 3.6 million. By 2013, only 37 percent of participants in the plans were actively employed.

The SOA’s report can be downloaded here.