CP 2019 ANNUAL REPORT /  66 Property, Plant and Equipment Deferred Income Taxes Personal Injury and Other Claims Liabilities Personal Injury The Company follows the group depreciation method under which a single depreciation rate is applied to the total cost in a particular class of property, despite differences in the service life or salvage value of individual properties within the same class. CP performs depreciation studies of each property asset class approximately every three years to update depreciation rates. The studies are conducted with assistance from third-party specialists and analyzed and reviewed by the Company's management. Depreciation studies for U.S. assets are reviewed and approved by the Surface Transportation Board ("STB"). Depreciation studies for Canadian assets are provided to the Canadian Transportation Agency (the "Agency"), but the Agency does not approve depreciation rates. In determining appropriate depreciation rates, management is required to make judgments and assumptions about a variety of key factors that are subject to future variability due to inherent uncertainties. These include the following: CP depreciates the cost of properties, net of salvage, on a straight-line basis over the estimated useful life of the class of property. The estimates of economic lives are uncertain and can vary due to changes in any of the assessed factors noted in the table above for whole and remaining asset lives. Additionally, the depreciation rates are updated to reflect the change in residual values of the assets in the class. It is anticipated that there will be changes in the estimates of weighted-average useful lives and net salvage for each property asset class as assets are acquired, used and retired. Substantial changes in either the useful lives of properties or the salvage assumptions could result in significant changes to depreciation expense. For example, if the estimated average life of track assets, including rail, ties, ballast and other track material, increased (or decreased) by one year, annual depreciation expense would decrease (or increase) by approximately $17 million. Due to the capital intensive nature of the railway industry, depreciation represents a significant part of operating expenses. The estimated useful lives of properties have a direct impact on the amount of depreciation recorded as a component of "Properties" on the Company’s Consolidated Balance Sheets. At December 31, 2019 and 2018, accumulated depreciation was $8,099 million and $7,964 million, respectively. CP accounts for deferred income taxes based on the liability method. This method focuses on the Company’s balance sheet and the temporary differences otherwise calculated from the comparison of book versus tax values. The provision for deferred income taxes arises from temporary differences in the carrying values of assets and liabilities for financial statement and income tax purposes and the effect of loss carry forwards. It is assumed that such temporary differences will be settled in the deferred income tax assets and liabilities at the balance sheet date. In determining deferred income taxes, the Company makes estimates and assumptions regarding deferred tax matters, including estimating the timing of the realization and settlement of deferred income tax assets (including the benefit of tax losses) and liabilities, and estimating unrecognized tax benefits for uncertain tax positions. Deferred income taxes are calculated using enacted federal, provincial, and state future income tax rates, which may differ in future periods. Deferred income tax expense is included in "Income tax expense" on the Company's Consolidated Statements of Income. Additional disclosures are provided in Item 8. Financial Statements and Supplementary Data, Note 6 Income taxes. CP estimates the potential liability arising from incidents, claims and pending litigations relating to personal injury claims by employees, third-party claims, certain occupation-related claims and property damage claims. In Canada, employee occupational injuries are governed by provincial workers' compensation legislation. Occupational injury claims in the provinces of Québec, Ontario, Manitoba and B.C. are self-insured and administered through each Worker's Compensation Board ("WCB"). The future costs related to occupation- related injuries are actuarially determined based on past experience and assumptions associated with the injury, compensation, income replacement, health care and administrative costs. In the four provinces where the Company is self-insured, a discount rate is applied to the future estimated costs based on market • • • • • • • • • • Key Assumptions Assessments Whole and remaining asset lives Statistical analysis of historical retirement patterns; Evaluation of management strategy and its impact on operations and the future use of specific property assets; Assessment of technological advances; Engineering estimates of changes in current operations and analysis of historic, current and projected future usage; Additional factors considered for track assets: density of traffic and whether rail is new or has been re-laid in a subsequent position; Assessment of policies and practices for the management of assets including maintenance; and Comparison with industry data. Salvage values Analysis of historical, current and estimated future salvage values.