9. Stock-Based Compensation

A summary of the Company’s outstanding stock option activity for the six months ended June 30, 2018 is as follows:

The Company estimates the fair value of each stock option on the grant date using the Black-Scholes-Merton valuation model, which requires several estimates including an estimate of the fair value of the underlying common stock on grant date. The expected volatility was based on an average of the historical volatility of a peer group of similar companies. The expected term was calculated utilizing the simplified method. The risk-free interest rate was based on the U.S. Treasury yield curve in effect at the time of grant for the expected term of the option.

Summary of Stock-Based Compensation Expense

Total stock-based compensation expense included in the condensed consolidated statements of operations is allocated as follows (in thousands):

Unrecognized stock-based compensation as of June 30, 2018 is as follows (in thousands):