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

	Three Months Ended
	April 3, 2020		March 29, 2019
Employee stock options	$	2,232	$	1,430
Restricted stock		78		82
Restricted stock units		549		1,104
Nonemployee stock options		62		25
Total stock-based compensation expense	$	2,921	$	2,641

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

	Three Months Ended
	April 3, 2020		March 29, 2019
Cost of sales	$	22	$	14
General and administrative		1,085		778
Marketing and selling		1,055		1,171
Research and development		759		678
Total stock-based compensation expense, net		2,921		2,641
Amounts capitalized as part of inventory		269		189
Total stock-based compensation expense, gross	$	3,190	$	2,830

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, and restricted stock units (“RSUs”). 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 outstanding under the Plan generally vest based on service, performance, or a combination of both. As of April 3, 2020, there were 844,399 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 3, 2020			March 29, 2019
Expected dividend yield		0	%		0	%
Expected volatility		53	%		53	%
Risk-free interest rate		0.54	%		2.43	%
Expected term (in years)		5.72			5.67

A summary of stock option activity under the Plan for the three months ended April 3, 2020 is presented below:

	Stock Options (in 000’s)			Minimum Exercise Price		Maximum Exercise Price
Outstanding at January 3, 2020		4,326
Granted		603
Exercised		(196	)
Forfeited or expired		(6	)
Outstanding at April 3, 2020		4,727		$	4.73	$	43.84
Exercisable at April 3, 2020		3,231

A summary of restricted stock and RSU activity under the Plan for the three months ended April 3, 2020 is presented below:

	Restricted Stock (in 000’s)		Restricted Stock Units (in 000’s)
Unvested at January 3, 2020		11		104
Granted		—		97
Vested		—		(87	)
Forfeited or expired		—		(1	)
Unvested at April 3, 2020		11		113