In August 2016, the Company’s board of directors adopted the 2016 Equity Incentive Plan (the 2016 Plan) for employees, directors, and consultants. As of December 31, 2018, 463,491 shares are available for issuance under the 2016 Plan, of which no shares were rolled from the 2013 Equity Incentive Plan (the 2013 Plan) into the 2016 Plan.

In 2013, the Company’s board of directors adopted the 2013 Plan for employees, directors, and consultants. The common stock authorized under the 2013 Plan is 1,200,000 shares. Under the Plan, the administrator shall have authority to determine which service providers will receive awards, to grant awards and to set all terms and conditions of awards (including, but not limited to, vesting, exercise and forfeiture provisions). Vested options are canceled 90 days after termination of employment and become available for reissuance under the Plan. As of December 31, 2018 and December 31, 2017, no shares were available for issuance under the 2013 Plan.

The service period for stock options granted to employees is generally 1 to 4 years.

The grant-date fair value of each option award is estimated on the date of grant using the Black- Scholes-Merton option-pricing model. The weighted average assumptions for grants during the years ended December 31, 2018, 2017 and 2016 are provided in the following table. The Company’s lack of historical share option exercise experience does not provide it a reasonable basis upon which to estimate an expected term because of a lack of sufficient data.

Therefore, the Company estimates the expected term by using the simplified method, which calculates the expected term as the average of the time-to-vesting and the contractual life of the options. Since the Company’s shares have only been publicly traded since August 12, 2016 and its shares were rarely traded privately, expected volatility is estimated based on the average historical volatility of similar entities with publicly traded shares in addition to the average historical volatility of the Company. The risk-free rate for the expected term of the option is based on the U.S. Treasury yield curve at the date of grant.

	As of December 31,
	2018			2017			2016
Valuation assumptions:
Expected dividend yield		0	%		0	%		0	%
Expected volatility		37.60	%		39.37	%		41.82	%
Expected term (years)		5.86			6.06			6.07
Risk-free interest rate		2.6	%		1.9	%		1.6	%

	Number of shares			Weighted average exercise price		Weighted average remaining contractual term
Balance at December 31, 2015		756,692		$	2.10		7.6
Granted		359,319			2.60		9.4
Exercised		(58,155	)		2.36		3.5
Expired/Forfeited		(17,469	)		2.13		2.6
Balance at December 31, 2016		1,040,387		$	2.25		7.8
Granted		471,144			14.72		9.1
Exercised		(284,455	)		2.07		5.2
Expired/Forfeited		(23,449	)		8.22		—
Balance at December 31, 2017		1,203,627		$	7.06		8.1
Granted		1,009,134			9.94		6.8
Exercised		(341,456	)		2.24		1.3
Expired/Forfeited		(464,256	)		11.81		—
Balance at December 31, 2018		1,407,049		$	8.73		8.1
Vested and exercisable at December 31, 2018		504,287		$	6.28		7.0
Vested and expected to vest at December 31, 2018		1,407,049		$	8.73		8.1

The weighted average grant-date fair value of options granted during the years ended December 31, 2018, 2017 and 2016 was $3.97, $6.02 and $1.23, respectively. For fully vested stock options the aggregate intrinsic value was $2,485,258, $3,596,624 and $7,770,086 as of December 31, 2018, 2017 and 2016, respectively. For stock options expected to vest the aggregate intrinsic value was $993,372, $1,469,154 and $4,569,243 as of December 31, 2018, 2017 and 2016, respectively. The total grant date fair value of shares vested during the years ended December 31, 2018, 2017 and 2016 was $2,770,008, $322,804 and $176,597, respectively.

During the year ended December 31, 2018, a total of 3,962 restricted stock units with a fair value of $12.62 per share were issued to a member of the Company’s Board of Directors of which the shares vest equally on each of the anniversaries over a three-year period. During the year ended December 31, 2016, a total of 57,475 shares of restricted common stock with a fair value of $2.00 per share were issued to the Company’s Chief Financial Officer and Chief Operating Officer of which 100% of the shares vested in the year ended December 31, 2017.

At December 31, 2018, 2017 and 2016, there was $2,781,062, $2,453,342 and $522,818, respectively, of total unrecognized compensation cost related to unvested stock options and restricted stock granted under the plans. That cost is expected to be recognized over the next three years.

The Company currently uses authorized and unissued shares to satisfy share award exercises.