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

	Three Months Ended				Six Months Ended
	July 3, 2020		June 28, 2019		July 3, 2020		June 28, 2019
Employee stock options	$	2,505	$	2,162	$	4,737	$	3,592
Restricted stock		76		77		154		159
Restricted stock units		274		311		823		1,415
Nonemployee stock options		63		29		125		54
Total stock-based compensation expense	$	2,918	$	2,579	$	5,839	$	5,220

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

	Three Months Ended				Six Months Ended
	July 3, 2020		June 28, 2019		July 3, 2020		June 28, 2019
Cost of sales	$	30	$	22	$	52	$	36
General and administrative		1,209		1,018		2,294		1,796
Marketing and selling		812		690		1,867		1,861
Research and development		867		849		1,626		1,527
Total stock-based compensation expense, net		2,918		2,579		5,839		5,220
Amounts capitalized as part of inventory		291		276		560		465
Total stock-based compensation expense, gross	$	3,209	$	2,855	$	6,399	$	5,685

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 July 3, 2020, there were 842,975 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						Six Months Ended
	July 3, 2020			June 28, 2019			July 3, 2020			June 28, 2019
Expected dividend yield		0	%		0	%		0	%		0	%
Expected volatility		53	%		53	%		53	%		53	%
Risk-free interest rate		0.31	%		2.05	%		0.53	%		2.41	%
Expected term (in years)		5.72			5.67			5.72			5.67

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

	Stock Options (in 000’s)			Minimum Exercise Price		Maximum Exercise Price
Outstanding at January 3, 2020		4,326
Granted		609
Exercised		(876	)
Forfeited or expired		(12	)
Outstanding at July 3, 2020		4,047		$	5.05	$	43.84
Exercisable at July 3, 2020		2,717

A summary of restricted stock and RSU activity under the Plan for the six months ended July 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		(11	)		(89	)
Forfeited or expired		—			(1	)
Unvested at July 3, 2020		—			111