The cost that has been charged against income for stock-based compensation is set forth below (in thousands):

	Three Months Ended
	April 2, 2021		April 3, 2020
Employee stock options	$	2,472	$	2,232
Restricted stock		166		78
Restricted stock units		537		549
Performance stock units		93		—
Nonemployee stock options		62		62
Total stock-based compensation expense	$	3,330	$	2,921

The Company recorded stock-based compensation costs in the following categories (in thousands):

	Three Months Ended
	April 2, 2021		April 3, 2020
Cost of sales	$	35	$	22
General and administrative		1,439		1,085
Marketing and selling		833		1,055
Research and development		1,023		759
Total stock-based compensation expense, net		3,330		2,921
Amounts capitalized as part of inventory		309		269
Total stock-based compensation expense, gross	$	3,639	$	3,190

The Amended and Restated Omnibus Equity Incentive Plan (“the Plan”) provides for various forms of stock-based incentives. To date, of the available forms of awards under the Plan, the Company has granted only stock options, restricted stock, unrestricted share grants, restricted stock units (“RSUs”) and performance stock units (“PSUs”). Options under the Plan are granted at fair market value on the date of grant, become exercisable generally over a three-year period, or as determined by the Board of Directors, and expire over periods not exceeding 10 years from the date of grant. Certain option and share awards provide for accelerated vesting if there is a change in control and pre-established financial metrics are met (as defined in the Plan). Grants of restricted stock outstanding under the Plan generally vest over periods of one to three years. Grants of RSUs and PSUs outstanding under the Plan generally vest based on service, performance, or a combination of both. As of April 2, 2021, there were 3,040,305 shares available for grant under the Plan.

The fair value of each option award is estimated on the date of grant using a Black-Scholes option valuation model applying the weighted-average assumptions noted in the following table. Expected volatilities are based on historical volatility of the Company’s stock. The expected term of options granted is derived from the historical exercises and post-vesting cancellations and represents the period of time that options granted are expected to be outstanding. The Company has calculated an 6% estimated forfeiture rate based on historical forfeiture experience. The risk-free rate is based on the U.S. Treasury yield curve corresponding to the expected term at the time of the grant.

	Three Months Ended
	April 2, 2021			April 3, 2020
Expected dividend yield		0	%		0	%
Expected volatility		53	%		53	%
Risk-free interest rate		0.84	%		0.54	%
Expected term (in years)		5.38			5.72

A summary of stock option activity under the Plan for the three months ended April 2, 2021 is presented below:

	Stock Options (in 000’s)			Minimum Exercise Price		Maximum Exercise Price
Outstanding at January 1, 2021		3,418
Granted		243
Exercised		(376	)
Forfeited or expired		—
Outstanding at April 2, 2021		3,285		$	5.34	$	106.51
Exercisable at April 2, 2021		2,324

A summary of restricted stock, RSUs and PSUs activity under the Plan for the three months ended April 2, 2021 is presented below:

	Restricted Stock (in 000’s)		Restricted Stock Units (in 000’s)			Performance Stock Units (in 000’s)
Unvested at January 1, 2021		11		122			15
Granted		—		48			—
Vested		—		(33	)		—
Forfeited or expired		—		—			—
Unvested at April 2, 2021		11		137			15