The corporation may prescribe by regulation actuarial assumptions which may be used by a plan actuary in determining the unfunded vested benefits of a plan for purposes of determining an employer’s withdrawal liability under this part. Withdrawal liability under this part shall be determined by each plan on the basis of—

(1) actuarial assumptions and methods which, in the aggregate, are reasonable (taking into account the experience of the plan and reasonable expectations) and which, in combination, offer the actuary’s best estimate of anticipated experience under the plan, or

(2) actuarial assumptions and methods set forth in the corporation’s regulations for purposes of determining an employer’s withdrawal liability.

In determining the unfunded vested benefits of a plan for purposes of determining an employer’s withdrawal liability under this part, the plan actuary may—

(1) rely on the most recent complete actuarial valuation used for purposes of section 412 of title 26 and reasonable estimates for the interim years of the unfunded vested benefits, and

(2) in the absence of complete data, rely on the data available or on data secured by a sampling which can reasonably be expected to be representative of the status of the entire plan.

For purposes of this part, the term “unfunded vested benefits” means with respect to a plan, an amount equal to—

(Pub. L. 93–406, title IV, § 4213, as added Pub. L. 96–364, title I, § 104(2), Sept. 26, 1980, 94 Stat. 1233; amended Pub. L. 101–239, title VII, § 7891(a)(1), Dec. 19, 1989, 103 Stat. 2445.)