Pension and Postretirement Benefit Plans Assumptions • Mortality rate  - This assumption reflects our best estimate, as of the measurement date, of the life expectancies of plan participants in order to determine the expected length of time for benefit payments. We derive our assumptions from industry mortality tables. • Long-term rate of return  - This assumption is selected from a range of probable return outcomes from an analysis of the asset portfolio. The market-related value as it relates to our estimate of long-term rate of return equals the fair value of plan assets, determined as of the measurement date. The return on plan assets recognizes all asset gains and losses, including changes in fair value, through the measurement date. Our expectations for the future investment returns of the asset categories are based on a combination of historical market performance, evaluations of investment forecasts obtained from external consultants and economists, and current market yields. The expected return for the total portfolio is calculated based on the plan's strategic asset allocation. The actual rate of return on plan assets is determined based on the fair value of the plan assets at the beginning and the end of the period, adjusted for contributions and benefit payments. A lower long- term rate of return on plan assets increases our net periodic benefit cost. • Discount rate  - This interest assumption is based on the yield derived from a portfolio of high quality fixed income corporate debt instruments that reasonably match the timing and amounts of projected future benefits for each of our retirement-related benefit plans. The rate is determined at the measurement date. A lower discount rate increases the present value of benefit obligations and increases our net periodic benefit cost. Investment risk is measured and monitored on an ongoing basis through annual liability measurements, periodic asset/liability studies, and quarterly investment portfolio reviews. Risk tolerance is established through consideration of plan liabilities, plan funded status, and corporate financial condition. We believe our investment portfolios are well diversified by asset class and sector, with no undue risk concentrations in any one category. See Note 9 of the "Notes to Consolidated Financial Statements" contained herein in Item 8 for further discussion of the investment portfolios for our plans. The assumptions chosen for our pension and OPEB plans are reviewed annually, using a December 31 measurement date for each of our plans unless we are required to perform an interim remeasurement. The discount rate, expected long-term rate of return, and mortality rate assumptions have the most significant effect on our net periodic benefit costs associated with these plans. In addition to the effect of changes in our assumptions, the net periodic cost or benefit obligation under our pension and OPEB plans may change due to factors such as plan amendments, actual experience being different from our assumptions, special benefits to terminated employees, and/or changes in benefits provided under the plans. Our net periodic benefit costs and the value of our benefit obligations for these plans are determined based on a set of economic and demographic assumptions that represent our best estimate of future expected experience. Major assumptions used in accounting for these plans include the expected discount (interest) rate, the long-term rate of return on plan assets, and mortality rates. We also use, as applicable, expected increases in compensation levels and a weighted average annual rate of increase in the per capita cost of covered benefits, which reflects a health care cost trend rate, and the U.K. pension plan also uses expected cost of living increases to plan benefits. We sponsor several defined benefit pension and other postretirement benefit (OPEB) plans for our employees, including non-qualified pension plans. The U.S. qualified and non-qualified defined benefit pension plans comprise the majority of our total benefit obligation and benefit cost. We maintain a separate defined benefit plan for eligible employees in our U.K. operation. The U.S. defined benefit pension plans were closed to new entrants on December 31, 2013, the OPEB plan was closed to new entrants on December 31, 2012, and the U.K. plan was closed to new entrants on December 31, 2002. 56