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Summary of Significant Accounting Policies (Tables) | 12 Months Ended
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Dec. 31, 2020
Accounting Policies [Abstract] |
Summary of common shares that were excluded from the diluted net loss per share calculation because of their anti-dilutive effect | The following table summarizes potential shares of common stock that were excluded from diluted net loss per share calculation because of their anti-dilutive effect:
| Year Ended December 31,
| 2020 |  | 2019 |  | 2018
Options to purchase common stock | 8,906,624 |  |  | 9,265,390 |  |  | 8,752,677 |
Service-based restricted stock units | 2,558,052 |  |  | 2,069,936 |  |  | 1,688,017 |
Performance-based restricted stock units | 663,353 |  |  | — |  |  | — |
Convertible preferred stock | 3,309 |  |  | 8,456 |  |  | 8,456 |
Convertible notes | 3,049,980 |  |  | 14,585,653 |  |  | — |
August 2019 Bonds | — |  |  | 3,799,071 |  |  | — |
December 2019 Bonds | 1,009,450 |  |  | 1,009,450 |  |  | — |
Total | 16,190,768 |  |  | 30,737,956 |  |  | 10,449,150 |
Summary of assumptions used to estimate the fair value of stock options | The weighted average assumptions used in the Black-Scholes model for option grants to employees and directors are presented below:
| Year Ended December 31,
| 2020 |  | 2019 |  | 2018
Risk-free interest rate | 0.63% |  | 2.42% |  | 2.73%
Expected volatility | 78% |  | 70% |  | 72%
Expected life in years | 6 |  | 6 |  | 6
Dividend yield | — |  | — |  | —
The weighted average assumptions used in the Black-Scholes model for option grants to non-employees are presented below:
| Year Ended December 31,
| 2020 |  | 2019 |  | 2018
Risk-free interest rate | 0.82% |  | 2.45% |  | 2.71%
Expected volatility | 76% |  | 88% |  | 78%
Expected life in years | 10 |  | 10 |  | 9
Dividend yield | — |  | — |  | —

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- Definition Tabular disclosure of securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) in the future that were not included in the computation of diluted EPS because to do so would increase EPS amounts or decrease loss per share amounts for the period presented, by antidilutive securities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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