Notice 2013-11 provides guidance on the 25-year average segment rates that are applied to adjust the otherwise applicable 24-month average segment rates that are used to compute the minimum contribution requirements for single-employer defined benefit (DB) plans.
Interest rates that are used for purposes of calculating the minimum required contribution are a set of three segment rates. Under MAP-21 (see “Congress Passes Bill with Pension Funding Relief”) these segment rates are adjusted to fall within a specified range that is determined based on a percentage of the average of the corresponding segment rates for the 25-year period ending on September 30 preceding the calendar year that includes the first day of that plan year. Under §430 of the Internal Revenue Code, for plan years beginning in 2013, each segment rate is adjusted so that it is no less than 85% and no more than 115% of the corresponding 25-year average segment rate.
For later plan years, this range is gradually expanded, so that the segment rates for plan years beginning after 2015 are no less than 70% and no more than 130% of the corresponding 25-year average segment rates. Notice 2013-11 is at http://www.irs.gov/pub/irs-drop/n-13-11.pdf.