29 U.S.C. § 1164

For purposes of this part— (1) In general The term “applicable premium” means, with respect to any period of continuation coverage of qualified beneficiaries, the cost to the plan for such period of the coverage for similarly situated beneficiaries with respect to whom a qualifying event has not occurred (without regard to whether such cost is paid by the employer or employee). (2) Special rule for self-insured plans To the extent that a plan is a self-insured plan— (A) In general Except as provided in subparagraph (B), the applicable premium for any period of continuation coverage of qualified beneficiaries shall be equal to a reasonable estimate of the cost of providing coverage for such period for similarly situated beneficiaries which— (i) is determined on an actuarial basis, and (ii) takes into account such factors as the Secretary may prescribe in regulations. (B) Determination on basis of past cost If an administrator elects to have this subparagraph apply, the applicable premium for any period of continuation coverage of qualified beneficiaries shall be equal to— (i) the cost to the plan for similarly situated beneficiaries for the same period occurring during the preceding determination period under paragraph (3), adjusted by (ii) the percentage increase or decrease in the implicit price deflator of the gross national product (calculated by the Department of Commerce and published in the Survey of Current Business) for the 12-month period ending on the last day of the sixth month of such preceding determination period. (C) Subparagraph (B) not to apply where significant change An administrator may not elect to have subparagraph (B) apply in any case in which there is any significant difference, between the determination period and the preceding determination period, in coverage under, or in employees covered by, the plan. The determination under the preceding sentence for any determination period shall be made at the same time as the determination under paragraph (3). (3) Determination period The determination of any applicable premium shall be made for a period of 12 months and shall be made before the beginning of such period.