Share-based Compensation Expense

Share-based compensation expense for all stock awards consists of the following (in millions):



	Years Ended
	December 31, 2017			January 1, 2017			January 3, 2016
Cost of product revenue	$	12		$	9		$	10
Cost of service and other revenue	2			2			2
Research and development	51			42			42
Selling, general and administrative	99			76			79
Share-based compensation expense before taxes	164			129			133
Related income tax benefits	(48		)	(41		)	(39		)
Share-based compensation expense, net of taxes	$	116		$	88		$	94

The assumptions used for the specified reporting periods and the resulting estimates of weighted-average fair value per share for stock purchased under the ESPP are as follows:



	Years Ended
	December 31, 2017			January 1, 2017			January 3, 2016
Risk-free interest rate	0.50% - 1.22%			0.40% - 0.50%			0.07% - 0.33%
Expected volatility	29% - 44%			40% - 44%			29% - 38%
Expected term	0.5 - 1.0 year			0.5 - 1.0 year			0.5 - 1.0 year
Expected dividends	0		%	0		%	0		%
Weighted-average grant-date fair value per share	$	46.81		$	48.29		$	53.92

As of December 31, 2017, approximately $394 million of total unrecognized compensation cost related to restricted stock and ESPP shares issued to date are expected to be recognized over a weighted-average period of approximately 2.7 years.