On June 18, 2015, our stockholders approved the 2015 Omnibus Equity Incentive Plan (“2015 OEIP”) and subsequent amendments to the 2015 OEIP to increase the number of shares reserved thereunder were approved by our stockholders on June 14, 2018 and June 9, 2020. The 2015 OEIP replaced the 2005 Stock Option / Stock Issuance Plan (“2005 Plan”) which was due to expire on July 28, 2015. All outstanding options under the 2005 Plan remain outstanding, but no new grants will be made under the 2005 Plan. The maximum number of shares of the Company’s common stock available for issuance over the term of the 2015 OEIP may not exceed 9,625,000 shares.

The 2015 OEIP provides for the issuance of full value awards (restricted stock, performance stock, dividend equivalent right or restricted stock units) and partial value awards (stock options or stock appreciation rights) to employees, non-employee members of the board and consultants. Any full value award settled in shares will be debited as 1.2 shares, and partial value awards settled in shares will be debited as 1.0 shares against the share reserve. The exercise price per share for stock option grants is not to be less than the fair market value per share of the Company’s common stock on the date of grant. The Board of Directors has the discretion to determine the vesting schedule. Stock options may be exercisable immediately or in installments, but generally vest over a four-year period from the date of grant. In the event the holder ceases to be employed by the Company, all unvested stock options terminate and all vested stock options may be exercised within a period of 90 days following termination. In general, stock options expire ten years from the date of grant. Restricted stock is valued using the closing stock price on the date of the grant. The total value is expensed over the vesting period of 12 to 48 months.

The Company has a shareholder approved employee stock purchase plan (“ESPP”), under which substantially all employees may purchase the Company’s common stock through payroll deductions at a price equal to 85% of the lower of the fair market values of the stock as of the beginning and end of six-month offering periods. An employee’s payroll deductions under the ESPP are limited to 10% of the employee’s compensation and employees may not purchase more than the lesser of $25,000 of stock, or 250 shares, for any purchase period. Additionally, no more than 250,000 shares in the aggregate may be purchased under the plan.

The Company accounts for all stock-based payment awards made to employees and directors based on their fair values and recognized as compensation expense over the vesting period using the straight-line method over the requisite service period for each award as required by FASB ASC Topic No. 718, Compensation-Stock Compensation.

The assumptions used to compute the share-based compensation costs for the stock options granted during the years ended December 31, 2020 and 2019, using the Black-Scholes option pricing model, were as follows:

	Year Ended December 31,
	2020			2019
Weighted average grant date fair value of stock options	$	2.93		$	1.84
Assumptions
Risk-free interest rate (weighted average)		0.44	%		2.36	%
Expected dividend yield		—			—
Weighted average expected life (years)		6.2			6.2
Volatility (weighted average)		80.8	%		78.7	%
Forfeiture rate		12.0	%		26.0	%

The risk-free interest rate assumption was based on the United States Treasury’s rates for U.S. Treasury zero-coupon bonds with maturities similar to those of the expected term of the award being valued. The Company assumed no dividend yield because it does not expect to pay dividends for the foreseeable future. The weighted average expected life is the vesting period for those options granted during that period. The average volatility is based on the actual historical volatility of our common stock. The forfeiture rate was based on modified employee turnover.

The fair values are estimated at the beginning of each offering period using a Black-Scholes valuation model that uses the assumptions noted in the following table. The risk-free rate is based on the U.S. treasury yield curve in effect at the time of grant. Expected volatility was based on the historical volatility on the day of grant. Following is a schedule of the shares purchased, the fair value per share, and the Black-Scholes model assumptions for each offering period:

	September 30,			March 31,			September 30,			March 31,
Offering Period Ended	2020			2020			2019			2019
Shares purchased for offering period		4,184			1,536			2,195			2,112
Fair value per share	$	1.75		$	2.28		$	1.07		$	0.96
Assumptions
Risk-free interest rate (average)		0.29	%		1.84	%		2.44	%		2.29	%
Expected dividend yield		—			—			—			—
Weighted average expected life (years)		0.5			0.5			0.5			0.5
Volatility (average)		86.8	%		86.3	%		51.7	%		54.3	%

Non-cash stock-based compensation expenses related to stock options, restricted stock grants and the ESPP were recorded in the financial statements as follows (in thousands):

A summary of the Company’s stock options outstanding under the 2015 OEIP and 2005 Plan as of December 31, 2020 and the activity during the years ended herein are as follows (in thousands except per share amounts):

	Shares			Weighted Avg. Exercise Price		Wtd. Avg. Remaining Contractual Life (Yrs)		Aggregate Intrinsic Value
Outstanding as of December 31, 2018		158		$	4.88		5.9	$	—
(121 options exercisable at a weighted average exercise price of $5.64)
Granted		65		$	2.65			$	—
Exercised		(13	)	$	3.81			$	28
Canceled / expired		(13	)	$	7.81			$	22
Outstanding as of December 31, 2019		197		$	4.03		6.3	$	171
(112 options exercisable at a weighted average exercise price of $5.16)
Granted		22		$	4.25			$	—
Exercised		(13	)	$	2.14			$	39
Canceled / expired		(1	)	$	42.04			$	—
Outstanding as of December 31, 2020		205		$	3.95		5.6	$	341

Exercisable as of December 31, 2020		135		$	4.35		4.1	$	183
Vested and expected to vest at December 31, 2020		181		$	3.77		5.5	$	304

As of December 31, 2020, there was $6.3 million of unrecognized compensation costs related to non-vested stock options and restricted stock granted under the Plans. At December 31, 2020, there were 5.1 million and 0 shares available for future grants under the 2015 OEIP and 2005 Plan, respectively.

A summary of the Company’s restricted stock awards outstanding under the 2015 OEIP and 2005 Plan as of December 31, 2020, and the activity during years ended therein, are as follows (in thousands):

				Weighted average
	Number					grant date
	of shares					fair value
Unvested at December 31, 2018		1,007			2.01
Granted		1,250			2.00
Vested		(673	)		2.06
Canceled and forfeited		(25	)		1.97
Unvested at December 31, 2019		1,559			1.98
Granted		1,000			6.40
Vested		(857	)		2.99
Canceled and forfeited		—			—
Unvested at December 31, 2020		1,702			4.07