Historically, stock options have been granted with an exercise price equal to the market value of the common stock on the date of grant, expire 10 years from the date they are granted, and generally vested over a one to three-year period for employees and members of the Board. Upon exercise, new shares will be issued by the Company. The restricted shares and restricted stock units (“RSUs”) granted to employees generally have a three-year graded vesting period and are subject to accelerated vesting and forfeiture under certain circumstances. Restricted shares and RSUs granted to Board members generally have a three-year graded vesting period and are subject to accelerated vesting and forfeiture under certain circumstances.

The following table provides the weighted average assumptions used in determining the fair value of the stock option awards granted during the three month periods ended March 31, 2020 and 2019.

The Company recognized approximately $0.4 million and $0.5 million of stock-based compensation expense during the three-month periods ended March 31, 2020 and 2019, respectively.