8. Stock‑Based Compensation

The Company maintains one stock incentive plan, the Second Amended and Restated 2004 Stock Incentive Plan (the “2004 Stock Incentive Plan”). The Company grants shares of restricted stock and deferred shares to key employees and stock awards to non‑employee members of the Company’s Board of Directors under the 2004 Stock Incentive Plan. Stock awards to non‑employee members of the Company’s Board of Directors vest immediately. Employees’ restricted stock awards and deferred shares typically vest over a three‑year period at the rate of one‑third per year. The restricted stock awards and deferred shares are amortized to expense on a straight-line basis over the vesting period. The Company issued 127,960 and 134,775 shares of restricted stock awards and deferred shares during the first nine months of 2017 and 2016, respectively.

The Company also grants performance stock units to key employees under the 2004 Stock Incentive Plan. Performance stock units typically vest at the end of a three-year performance period set by the Compensation Committee of the Board of Directors at the time of grant. Upon vesting, the number of shares of the Company’s Class A common stock awarded to each performance stock unit recipient will be determined based on the Company’s performance relative to certain performance goals set at the time the performance stock units were granted. The recipient of a performance stock unit award may earn from zero shares to twice the number of target shares awarded to such recipient. The performance stock units are amortized to expense over the vesting period, and based on the Company’s performance relative to the performance goals, may be adjusted. Changes to the estimated shares expected to vest will result in adjustments to the related share-based compensation expense that will be recorded in the period of change. If the performance goals are not met, no awards are earned and previously recognized compensation expense is reversed. The Company granted 98,812 and 106,724 of annual awards for performance stock units during the first nine months of 2017 and 2016, respectively. The performance goals for the performance stock units are based on the compound annual growth rate of the Company’s revenue over the three-year performance period and the Company’s return on invested capital (“ROIC”) for the third year of the performance period.

The Company also has a Management Stock Purchase Plan that allows for the granting of restricted stock units (RSUs) to key employees. On an annual basis, key employees may elect to receive a portion of their annual incentive compensation in RSUs instead of cash. Participating employees may use up to 50% of their annual incentive bonus to purchase RSUs for a purchase price equal to80% of the fair market value of the Company’s Class A common stock as of the date of grant. Upon vesting, each RSU is converted into one share of Class A common stock. Beginning with annual incentive compensation for 2016, the purchase price for RSUs was increased from 67% to 80% of the fair market value of the Company’s Class A common stock. RSUs vest either annually over a three-year period from the grant date or upon the third anniversary of the grant date and receipt of the shares underlying RSUs is deferred for a minimum of three years, or such greater number of years as is chosen by the employee, from the date of grant. An aggregate of 2,000,000 shares of Class A common stock may be issued under the Management Stock Purchase Plan. The company granted 47,222 RSU’s and 88,882 RSU’s during the first nine months of 2017 and 2016, respectively.

The fair value of each RSU issued under the Management Stock Purchase Plan is estimated on the date of grant, using the Black‑Scholes‑Merton Model, based on the following weighted average assumptions:


	2017		2016
Expected life (years)	3.0		3.0
Expected stock price volatility	25.0	%	24.8	%
Expected dividend yield	1.2	%	1.3	%
Risk-free interest rate	1.5	%	0.9	%

The risk‑free interest rate is based upon the U.S. Treasury yield curve at the time of grant for the respective expected life of the RSUs. The expected life (estimated period of time outstanding) of RSUs and volatility were calculated using historical data. The expected dividend yield of stock is the Company’s best estimate of the expected future dividend yield.

The above assumptions were used to determine the weighted average grant‑date fair value of RSUs granted of $16.84 and $18.15 in 2017 and 2016, respectively.

A more detailed description of each of these plans can be found in Note 12 of the Notes to Consolidated Financial Statements in the Company’s Annual Report on Form 10-K for the year ended December 31, 2016.