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STOCK-BASED COMPENSATION (Tables) | 12 Months Ended
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Dec. 31, 2021
Schedule of stock-based compensation | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Years Ended December 31,
​ |  | 2021 |  | 2020 |  | 2019
Equity classified awards | ​ | $ | 15,428 | ​ | $ | 12,272 | ​ | $ | 7,327
Liability classified awards | ​ | ​ | 311 | ​ | ​ | — | ​ | ​ | —
Stock-based compensation expense | ​ | $ | 15,739 | ​ | $ | 12,272 | ​ | $ | 7,327
Schedule of changes in unvested RSUs | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended December 31, 2021
​ |  | ​ |  | Weighted-
​ | ​ | ​ | ​ | Average
​ | ​ | Number of | ​ | Grant Date
​ | ​ | RSUs | ​ | Fair Value
RSUs outstanding at beginning of period |  | 608 | ​ | $ | 58.15
RSUs granted |  | 406 | ​ | $ | 94.60
RSUs vested |  | (178) | ​ | $ | 62.82
RSUs forfeited |  | (209) | ​ | $ | 70.34
RSUs outstanding at end of period |  | 627 | ​ | $ | 76.37
Summary of changes in stock options | Changes in our stock options were as follows: ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended December 31, 2021
​ |  | ​ |  | Weighted-
​ | ​ | ​ | ​ | Average
​ | ​ | Number of | ​ | Exercise Price
​ | ​ | Options | ​ | per Share
Options outstanding at beginning of period |  | 147 | ​ | $ | 23.63
Options exercised |  | (35) | ​ | $ | 21.10
Options outstanding at end of period |  | 112 | ​ | $ | 24.41
Schedule of shares authorized under stock option plans, by exercise price range | The following table summarizes information about the stock options outstanding on December 31, 2021: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Options Outstanding and Exercisable
​ | ​ | Number | ​ | Weighted-Average | ​ | Weighted-
​ | ​ | Outstanding | ​ | Remaining | ​ | Average
Range of Exercise Prices |  | (In 000's) |  | Contractual Life |  | Exercise Price
$18.77 |  | 28 |  | 2.75 years | ​ | $ | 18.77
$26.32 |  | 84 |  | 3.10 years | ​ | $ | 26.32
$18.77 to $26.32 |  | 112 |  | 3.01 years | ​ | $ | 24.41
Employee Stock [Member] |
Schedule of stock-based compensation |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Years Ended December 31,
​ |  | 2021 |  | 2020 |  | 2019
Stock-based compensation expense related to the ESPP | ​ | $ | 1,114 | ​ | $ | 856 | ​ | $ | 475
​
Schedule of valuation assumptions | The fair value of each purchase right granted under the ESPP was estimated on the date of grant using the Black-Scholes-Merton option pricing model with the following assumptions: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Years Ended December 31, | ​
​ |  | 2021 |  | 2020 |  | 2019 |
Risk-free interest rates |  | 0.04% - 0.10 | % | 0.10% - 0.18% | % | 1.62% - 2.31 | %
Expected dividend yield rates |  | — | % | — | % | — | %
Expected term |  | 0.5 years |  | 0.5 years |  | 0.5 years | ​
Expected volatility |  | 42.7 | % | 70.1 | % | 41.3 | %

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