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Equity (Tables)
 12 Months Ended
Dec. 31, 2021
Schedule of assumptions used to determine fair value of stock options granted
The fair value of the stock options granted during each of the years ended December 31, 2021, 2020 and 2019 was determined using the Black-Scholes option pricing model at the grant date with the following range of assumptions:
Year ended December 31,
202120202019
Expected volatility
74% - 78%
70% - 75%
69% - 71%
Expected term (years)
5.3 - 7.0
5.3 - 7.0
5.5 - 7.0
Risk free interest rate
0.5% - 1.3%
0.3% - 1.6%
1.4% - 2.7%
Expected dividend yield0 %0 %0 %
Exercise price
$66.82 - $100.40
$22.70 - $50.00
$17.29 - $37.66
Summary of stock option activity
A summary of the stock option activity under the 2018 Plan as of December 31, 2021 is presented below. These amounts include stock options granted to employees, directors and consultants.
(dollars in millions,
except weighted average exercise price)		OptionsWeighted
Average
Exercise Price		Weighted
Average
Remaining
Contractual
Term (Years)		 Aggregate
Intrinsic Value
Outstanding at December 31, 2020
4,321,882 $26.35 
Granted1,866,659 $79.24 
Exercised(773,476)$22.99 
Forfeited(71,811)$50.72 
Outstanding at December 31, 2021
5,343,254 $44.98 8.1$200.4 
Exercisable at December 31, 2021
2,289,309 $23.62 7.2$134.0 
Restricted stock awards  
Summary of restricted stock award activity
A summary of the restricted stock award activity under the Incentive Plan as of December 31, 2021 is presented below. These amounts include restricted stock granted to employees, directors and consultants.
SharesWeighted
Average Grant
Date
Fair Value Per
Share
Unvested restricted stock at December 31, 2020
238,712 $16.00 
Vested(208,087)$16.00 
Unvested restricted stock at December 31, 2021
30,625 $16.00 
Restricted stock units  
Summary of restricted stock award activity
A summary of restricted stock unit activity under the 2018 Plan for the year ended December 31, 2021 is presented below. These amounts include restricted stock units granted to employees.
SharesWeighted
Average Grant
Date
Fair Value Per
Share
Unvested restricted stock units at December 31, 2020
133,049 $20.01 
Exercised(44,355)$20.01 
Forfeited(387)$19.36 
Unvested restricted stock units at December 31, 2021
88,307 $20.02