Assumptions Periodic pension and OPEB cost is actuarially calculated annually for each individual plan based on data available at the beginning of each year. Assumptions used in the calculation include the settlement discount rate selected and disclosed at the end of the previous year as well as other assumptions detailed in the table below. The weighted-average rates used in determining the periodic pension and OPEB cost for the fiscal years 2018, 2017 and 2016 were as follows: Benefit obligations are actuarially calculated annually at the end of each year based on the assumptions detailed in the table below. Obligations under the OPEB plans are determined by the application of the terms of medical, dental, vision and life insurance plans, together with relevant actuarial assumptions and heath care cost trend rates. The weighted-average rates used in determining the projected benefit obligation for defined pension plans and the accumulated postretirement benefit obligation for OPEB plans, as of December 31, 2018, and December 31, 2017, were as follows: The change to the weighted-average discount rates used for our defined benefit pension plans and postretirement plans as of December 31, 2018, from December 31, 2017, is primarily the result of overall market changes during 2018. The obligations of our defined benefit pension plans in the U.S., Canada and the U.K. are supported by assets held in trusts for the payment of future benefits. The business segments are obligated to adequately fund these asset trusts. The underlying investments within our defined benefit pension plans include: cash and short-term instruments, debt securities, equity securities, investment funds, and other investments including derivatives, hedge fund of funds and real estate. Investment allocations reflect the customized strategies of the respective plans. The plans use liability driven investment strategies in managing defined pension benefits. For all defined benefit pension plan assets the plans have the following primary investment objectives: Investment Strategy 140 As of December 31, 2018 As of December 31, 2017 Pension OPEB Pension OPEB Weighted-average assumptions: Settlement discount rate 3.44% 3.92% 3.01% 3.34% Rate of compensation increase 2.00% N/A 2.00% N/A Health care cost trend rate N/A Ranging ratably from 6.5% in 2019 to 4.5% in 2037 N/A Ranging ratably from 6.75% in 2018 to 4.5% in 2037 For the years ended December 31, 2018 December 31, 2017 December 31, 2016 Pension OPEB Pension OPEB Pension OPEB Weighted-average assumptions: Settlement discount rate 3.01% 3.34% 3.36% 3.76% 3.72% 3.59% Rate of compensation increase 2.00% N/A 2.00% N/A 2.00% N/A Expected return on plan assets (1) 4.10% N/A 4.83% N/A 5.15% N/A Health care cost trend rate N/A Ranging ratably from 6.75% in 2018 to 4.5% in 2037 N/A Ranging ratably from 7.0% in 2017 to 4.5% in 2037 N/A Ranging ratably from 7.7% in 2016 to 4.5% in 2028 (1) We develop our EROA assumptions annually with input from independent investment specialists including our actuaries, investment consultants, plan trustee and other specialists. Each EROA assumption is based on historical data, including historical returns, historical market rates and is calculated for each plan's individual asset class. The calculation includes inputs for interest, inflation, credit, and risk premium (active investment management) rates and fees paid to service providers. We consider our EROA to be a significant management estimate. Any material changes in the inputs to our methodology used in calculating our EROA could have a significant impact on our reported defined benefit pension plans' expense.