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Stock-Based Compensation | 12 Months Ended
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Dec. 31, 2019
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
Stock-Based Compensation | NOTE 9 – STOCK-BASED COMPENSATION The following table shows total stock-based compensation expense recognized in the accompanying Consolidated Statements of Operations (in thousands):
| Year Ended December 31, |  |  |  |
| 2019 |  | 2018 |  | 2017
Research and development | $ | 10,135 |  |  | $ | 8,249 |  |  | $ | 5,031 |
General and administrative | 9,195 |  |  | 6,428 |  |  | 4,058 |
Total stock-based compensation expense | $ | 19,330 |  |  | $ | 14,677 |  |  | $ | 9,089 |
As of December 31, 2019, total stock-based compensation expense to be recognized in future periods related to unvested stock options was $37.4 million, which is expected to be expensed over a weighted-average period of 2.76 years. As of December 31, 2019, total stock-based compensation expense to be recognized in future periods related to unvested RSUs was $7.0 million, which is expected to be expensed over a weighted-average period of 2.11 years. There was no capitalized stock-based employee compensation expense as of December 31, 2019, 2018 or 2017. Valuation Assumptions Employee stock-based compensation expense was determined using the Black-Scholes option valuation model for stock options and employee share purchases under the ESPP. Option valuation models require the input of subjective assumptions and these assumptions can vary over time. The fair value of RSUs was based on the closing price of the underlying common stock on the date of grant. The Company bases its determination of expected volatility through its assessment of the historical volatility of its common stock. The Company relied on its historical exercise and post-vested termination activity for estimating its expected term for use in determining the fair value of these options. The weighted-average estimated fair value per share of options granted during 2019, 2018 and 2017 was $6.37, $11.39, and $4.10, respectively, based upon the assumptions used in the Black-Scholes valuation model. The assumptions used for estimating the fair value of the employee stock options were as follows:
| Year Ended December 31, |  |  |  |
| 2019 |  | 2018 |  | 2017
Risk-free interest rate | 1.68-2.25% |  |  | 2.53-2.96% |  |  | 1.81-2.28%
Expected term (in years) | 5.50-5.62 |  | 5.59-5.61 |  | 5.73-5.83
Expected dividend yield of stock | — |  |  | — |  |  | — |
Expected volatility | 76.46-78.39% |  | 72.33-75.49% |  | 71.11-72.30%
Employees purchased 249,364, 328,710 and 253,994 shares of common stock through the ESPP at an average exercise price of $8.53, $4.51, and $3.22 per share during 2019, 2018 and 2017, respectively. The weighted-average estimated fair values of shares purchased under the Company’s ESPP during 2019, 2018 and 2017 were $4.70, $7.07 and $2.37, respectively, based upon the assumptions used in the Black-Scholes valuation model. The weighted–average assumptions used for estimating the fair value of the ESPP purchase rights are as follows:
| Year Ended December 31, |  |  |  |
| 2019 |  | 2018 |  | 2017
Risk-free interest rate | 1.53-2.42% |  | 2.16-2.80% |  | 0.44-0.76%
Expected term (in years) | 0.5 – 2.0 |  | 0.5-2.0 |  | 0.5-2.0
Expected dividend yield of stock | — |  |  | — |  |  | — |
Expected volatility | 51.02-91.96% |  | 73.21-83.25% |  | 66.39-82.19%

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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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