The fair value assigned to the vested increments of these awards was estimated at the grant date of the stock options using the Black-Scholes option-pricing model with the following assumptions:
| Year ended December 31, 2016 | Year ended December 31, 2015 | |||||||
| Stock price | $1.80 - $3.55 | $1.25 - $4.00 | ||||||
| Risk-free interest rate | 0.94% - 2.10% | 1.18% - 1.81% | ||||||
| Expected term (in years) | 5.0 | 5.0 | ||||||
| Weighted average expected stock price volatility | 34.28% | 71.32% | ||||||
| Expected dividend yield | 0% | 0% | ||||||
| Weighted average grant-date fair value of options granted | $ | 1.0539 | $ | 2.7592 | ||||