9. Stock-Based Compensation

A summary of the Company’s outstanding stock option activity for the years ended December 31, 2017 and 2016 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. The following weighted average assumptions were used in the calculation to estimate the fair value of options granted to employees during the year ended December 31, 2016, (no options were granted for the year ended December 31, 2017):

Summary of Stock-Based Compensation Expense

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

Unrecognized stock-based compensation at December 31, 2016 and 2017 were as follows (in thousands):