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Stock-Based Compensation | 3 Months Ended
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Mar. 31, 2019
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
Stock-Based Compensation | Stock-Based Compensation The Company incurs stock-based compensation expense related to restricted stock units and stock options. The fair value of restricted stock is determined by the closing price of the Company's common stock reported on the Nasdaq Global Select Market on the date of grant. The Company estimates the fair value of stock options granted using the Black-Scholes option pricing model. The Black-Scholes option pricing model was developed for use in estimating the fair value of traded options, which have no vesting restrictions and are fully transferable. In addition, option valuation models require the input of subjective assumptions, including the expected stock price volatility and expected option life. The Company amortizes the fair value of the awards on a straight-line basis over the requisite vesting period of the awards. Expected volatility is based on historical volatility. The expected life of options granted is based on historical expected life. The risk-free interest rate is based on the U.S. Treasury yield in effect at the time of grant. The dividend yield is based on the fact that no dividends have been paid historically and none are currently expected to be paid in the foreseeable future. The weighted average assumptions used in the Black-Scholes model for option grants to employees and directors are presented below:
| Three Months Ended March 31,
| 2019 |  | 2018
Risk-free interest rate | 2.44% |  | 2.72%
Expected volatility | 70% |  | 72%
Expected life in years | 6.2 |  | 6.2
Dividend yield | — |  | —
Total employee and director stock-based compensation expense recognized in the condensed consolidated statements of operations for the three months ended March 31, 2019 and 2018 was $3.2 million and $3.4 million, respectively, of which $1.9 million and $2.1 million, respectively, were included in research and development expenses, and $1.3 million and $1.3 million, respectively, were included in general and administrative expenses. At March 31, 2019, there was $7.3 million of total unrecognized compensation expense related to unvested stock options, which is expected to be recognized over a weighted-average period of 2.2 years. The weighted average grant date fair value per share, calculated using the Black-Scholes option pricing model, was $2.19 and $2.84 for employee and director stock options granted during the three months ended March 31, 2019 and 2018, respectively. At March 31, 2019, there was $6.1 million of total unrecognized compensation expense related to unvested restricted stock units, which is expected to be recognized over a weighted-average period of 2.1 years. The weighted average grant date fair value per share was $3.38 and $4.29 for restricted stock units granted during the three months ended March 31, 2019 and 2018, respectively. The Company adopted ASU 2018-07 on January 1, 2019 which generally aligned the accounting for stock-based compensation for non-employees with that of employees. The fair value of stock options granted to non-employees was estimated using the Black-Scholes pricing model. Total stock-based compensation expense for stock options and restricted stock units granted to non-employees for the three months ended March 31, 2019 and 2018 was $274,000 and $140,000, respectively.

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- Definition The entire disclosure for compensation-related costs for equity-based compensation, which may include disclosure of policies, compensation plan details, allocation of equity compensation, incentive distributions, equity-based arrangements to obtain goods and services, deferred compensation arrangements, employee stock ownership plan details and employee stock purchase plan details. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -URI http://asc.fasb.org/topic&trid=2228938 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 50 -URI http://asc.fasb.org/subtopic&trid=2208855 + Details
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