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

	Three Months Ended
	March 29, 2019		March 30, 2018
Employee stock option	$	1,430	$	619
Restricted stock		82		51
Restricted stock units		1,104		598
Nonemployee stock options		25		33
Total stock-based compensation expense	$	2,641	$	1,301

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

	Three Months Ended
	March 29, 2019		March 30, 2018
Cost of sales	$	14	$	3
General and administrative		778		519
Marketing and selling		1,171		460
Research and development		678		319
Total stock-based compensation expense, net		2,641		1,301
Amounts capitalized as part of inventory		189		122
Total stock-based compensation expense, gross	$	2,830	$	1,423

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 March 29, 2019, there were 1,694,616 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 8% 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
	March 29, 2019			March 30, 2018
Expected dividend yield		0	%		0	%
Expected volatility		53	%		53	%
Risk-free interest rate		2.43	%		2.61	%
Expected term (in years)		5.67			5.72

A summary of stock option activity under the Plan for the three months ended March 29, 2019 is presented below:

	Stock Options (in 000’s)			Minimum Exercise Price		Maximum Exercise Price
Outstanding at December 28, 2018		3,920
Granted		749
Exercised		(74	)
Forfeited or expired		(1	)
Outstanding at March 29, 2019		4,594		$	1.92	$	43.84
Exercisable at March 29, 2019		2,839

A summary of restricted stock and RSU activity under the Plan for the three months ended March 29, 2019 is presented below:

	Restricted Stock (in 000’s)		Restricted Stock Units (in 000’s)
Unvested at December 28, 2018		11		322
Granted		—		19
Vested		—		(178	)
Forfeited or expired		—		(1	)
Unvested at March 29, 2019		11		162