Stock-based Compensation (Tables)

The following table summarizes stock-based compensation costs for the years ended December 31, 2016, 2015 and 2014.



	Year Ended December 31
Millions of dollars	2016			2015			2014
Stock-based compensation cost	$	262		$	294		$	298
Tax benefit	(77		)	(99		)	(90		)
Stock-based compensation cost, net of tax	$	185		$	195		$	208

The assumptions and resulting fair values of options granted were as follows:



	Year Ended December 31
	2016	2015	2014
Expected term (in years)	5.21	5.16	5.23
Expected volatility	37%	39%	37%
Expected dividend yield	1.35 - 2.46%	1.51 - 1.85%	0.94 - 1.77%
Risk-free interest rate	1.13 - 1.84%	1.43 - 1.72%	1.57 - 1.86%
Weighted average grant-date fair value per share	$12.33	$13.47	$19.26

The following table represents our stock options activity during 2016.



	Number of Shares (in millions)		Weighted Average Exercise Price per Share		Weighted Average Remaining Contractual Term (years)	Aggregate Intrinsic Value (in millions)
Outstanding at January 1, 2016	20.0		$	43.90
Granted	3.8		41.18
Exercised	(2.2	)	35.93
Forfeited/expired	(1.0	)	49.06
Outstanding at December 31, 2016	20.6		$	44.01	6.8	$	239
Exercisable at December 31, 2016	13.0		$	43.26	5.7	$	161

The following table represents our restricted stock awards and restricted stock units granted, vested and forfeited during 2016.



	Number of Shares (in millions)		Weighted Average Grant-Date Fair Value per Share
Nonvested shares at January 1, 2016	16.5		$	45.59
Granted	5.5		42.87
Vested	(5.3	)	44.43
Forfeited	(1.6	)	46.05
Nonvested shares at December 31, 2016	15.1		$	44.96

The assumptions and resulting fair values were as follows:



	Year Ended December 31
	2016			2015			2014
Expected volatility	36		%	35		%	23		%
Expected dividend yield	1.87		%	1.82		%	1.07		%
Risk-free interest rate	0.25		%	0.01		%	0.04		%
Weighted average grant-date fair value per share	$	8.61		$	8.62		$	11.80