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Stock Options and Employee Stock Purchase Plan (Tables)

9 Months Ended

Sep. 30, 2021

The following table represents stock option activity for the nine months ended September 30, 2021:


	Number of Shares	Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Outstanding at January 1, 2021	475,601	$	69.61
Granted	—	—
Exercised	(68,601)	42.08
Canceled	—	—
Outstanding at September 30, 2021	407,000	$	74.25	6.2	$	25,385,630
Exercisable at September 30, 2021	157,000	$	73.00	6.0	$	9,988,130
Vested and expected to vest at September 30, 2021	346,640	$	74.11	6.1	$	21,668,058

The weighted-average fair values of market-based shares granted have been estimated utilizing the following assumptions:


	September 30, 2021			September 30, 2020
Underlying stock price at valuation date	$	113.27		$	91.17
Expected volatility	30.3		%	27.0		%
Risk-free interest rate	1.3		%	0.7		%

Restricted stock award activity for the nine months ended September 30, 2021 is set forth below:


	Shares	Weighted-Average Grant-Date Fair Value
Nonvested at January 1, 2021	820,566	$	62.66
Granted	—	—
Vested	(421,923)	62.47
Canceled	(3,277)	82.96
Nonvested at September 30, 2021	395,366	$	61.62

Restricted stock unit award activity for the nine months ended September 30, 2021 is set forth below:


	Number of Shares	Weighted-Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value
Outstanding at January 1, 2021	209,784
Granted	164,257
Vested	(124,761)
Canceled	(3,262)
Outstanding at September 30, 2021	246,018	3.2	$	33,610,979
Vested and expected to vest at September 30, 2021	170,143	2.5	$	23,244,890

The compensation expense related to the Purchase Plan has been estimated utilizing the following assumptions:


	September 30, 2021	September 30, 2020
Risk-free interest rate	0.06%	0.13%
Expected term (in years)	0.5	0.5
Dividend yield	0.00%	0.00%
Expected volatility	35.62%	23.95%
Weighted average volatility	35.62%	23.95%