8.	Stock-Based Compensation

For stock-based awards granted by the Company, stock-based compensation expense is measured at grant date based on the fair value of the award and is expensed over the requisite service period. The Company recorded stock-based compensation expense of $762 and $1,030 for the three months ended June 30, 2019 and 2018, respectively, and $1,447 and $2,058 for the six months ended June 30, 2019 and 2018, respectively.

Stock Options

The Company uses the Black-Scholes option-pricing model to estimate the fair value of options. This model requires the input of highly subjective assumptions including the expected volatility, risk-free interest rate and the expected life of options. The Company used the following assumptions for its Black-Scholes option-pricing model for the periods presented:

	Three Months Ended June 30,		Six Months Ended June 30,
	2019	2018	2019	2018
Dividend yield	—	—	—	—
Expected volatility	66.4%	55.7%	66.4%	55.9%
Risk-free interest rate	2.18%	2.56%	2.18%	2.54%
Expected life of options (in years)	4.00	4.00	4.00	4.00

The Company estimates the expected volatility of its common stock and expected life of its stock options based on its own historical experience. The expected volatility reflects the actual historical volatility of the price of the Company’s common stock since it began trading publicly in March 2013. The expected life represents the period of time that stock options are expected to be outstanding, based on historical exercise and employee departure behavior. The Company has no history or expectation of paying cash dividends on its common stock. The risk-free interest rate is based on the U.S. Treasury yield for a term consistent with the expected life of the options in effect at the time of grant.

There were no exercises of stock options during the three and six months ended June 30, 2019 and 2018.

Compensation expense, net of estimated forfeitures, is recognized ratably over the requisite service period. As of June 30, 2019, there was $555 of unrecognized compensation expense related to stock options, which is expected to be recognized over a weighted-average period of 1.9 years.

RSUs

As of June 30, 2019, there was $4,960 of unrecognized compensation expense, net of estimated forfeitures, related to RSUs, which is expected to be recognized over a weighted-average period of 2.6 years. The Company uses the fair market value of the underlying common stock on the dates of grant to determine the fair value of RSUs.

Employee Stock Purchase Plan

The Company estimates the fair value of purchase rights under the 2013 ESPP using the Black-Scholes valuation model. The fair value of each purchase right under the 2013 ESPP is estimated on the date of grant using the Black-Scholes option valuation model and the straight-line attribution approach with assumptions substantially similar to those used for the valuation of stock option awards, with the exception of the expected life. The expected life is estimated to be six months, which is consistent with the purchase periods under the 2013 ESPP.