Stock-based compensation:
We generally grant stock options, performance share units (PSUs) and restricted share units (RSUs) to employees under our stock-based compensation plans. Stock options and RSUs vest in installments over the vesting period. Stock options generally vest 25% per year over a four-year period, and RSUs generally vest one-third per year over a three-year period. We treat each installment under a grant of stock options and RSUs as a separate grant in determining the compensation expense. PSUs vest at the end of their respective terms, generally three years from the grant date, to the extent that specified performance conditions have been met.
Options are exercisable for subordinate voting shares. We recognize the grant date fair value of options granted to employees as compensation expense in our consolidated statement of operations, with a corresponding charge to contributed surplus on our consolidated balance sheet, over the vesting period. We adjust compensation expense to reflect the estimated number of options we expect to vest at the end of the vesting period. When options are exercised, we credit the proceeds to capital stock on our consolidated balance sheet. We measure the fair value of options using the Black-Scholes option pricing model. Measurement inputs include the price of our subordinate voting shares on the grant date, the exercise price of the option, and our estimates of the following: expected price volatility of our subordinate voting shares (based on weighted average historic volatility), weighted average expected life of the option (based on historical experience and general option holder behavior), and the risk-free interest rate. The cost we record for RSUs and 40% of PSUs granted annually is based on the market value of our subordinate voting shares at the time of grant. The cost we record for these PSUs, which vest based on a non-market performance condition related to the achievement of pre-determined financial targets over a specified period, is based on our estimate of the outcome of such performance condition. We adjust the cost of these PSUs as new facts and circumstances arise; the timing of these adjustments is subject to judgment. We generally record adjustments to the cost of these PSUs during the last year of the three-year term based on management's estimate of the expected level of achievement of such performance condition. We amortize the cost of RSUs and these PSUs to compensation expense in our consolidated statement of operations, with a corresponding charge to contributed surplus on our consolidated balance sheet, over the vesting period. We generally settle these awards with subordinate voting shares purchased in the open market by a broker, or by issuing subordinate voting shares from treasury.
We determine the cost we record for 60% of PSUs granted annually using a Monte Carlo simulation model. The number of awards expected to vest is factored into the grant date Monte Carlo valuation for the award. The number of these PSUs that will vest depends on the level of achievement of total shareholder return (TSR), which is a market performance condition, relative to the TSR of a pre-defined group of companies over a three-year period. We do not adjust the grant date fair value regardless of the eventual number of awards that vest based on the level of achievement of the market performance condition. We recognize compensation expense in our consolidated statement of operations on a straight-line basis over the requisite service period and we reduce this expense for the estimated PSU awards that are not expected to vest because the employment conditions are not expected to be satisfied.
The compensation of our Board of Directors in 2017 was comprised of annual Board retainer fees, annual Audit and Compensation Committee Chair retainer fees (for the Chairs of those committees) and travel fees (collectively, Annual Fees) payable in quarterly installments in arrears. Directors must elect to have either 75% or 100% of their Annual Fees paid in deferred share units (DSUs). The number of DSUs we grant is determined by dividing the elected percentage of the dollar value of the Annual Fees earned in the quarter by the closing price of our subordinate voting shares on the New York Stock Exchange (NYSE) on the last business day of such quarter. Each DSU represents the right to receive one subordinate voting share or an equivalent value in cash after the individual ceases to serve as a director. For DSUs granted prior to January 1, 2007, we may settle these share units with subordinate voting shares issued from treasury or purchased in the open market, or with cash (at the discretion of the Company). For DSUs granted after January 1, 2007, we may only settle these share units with subordinate voting shares purchased in the open market, or with cash (at the discretion of the Company). We expense the cost of DSUs through SG&A in our consolidated statement of operations in the period the services are rendered.
