Stock-based Compensation (Tables)

The following table summarizes stock-based compensation costs for the years ended December 31, 2013, 2012, and 2011.



	Year Ended December 31
Millions of dollars	2013			2012			2011
Stock-based compensation cost	$	264		$	217		$	198
Tax benefit	(81		)	(67		)	(61		)
Stock-based compensation cost, net of tax	$	183		$	150		$	137

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



	Year Ended December 31
	2013	2012	2011
Expected term (in years)	5.27	5.21	5.20
Expected volatility	40%	46%	40%
Expected dividend yield	0.94 - 1.33%	0.99 – 1.24%	0.69 – 1.01%
Risk-free interest rate	0.77 - 1.73%	0.65 – 1.15%	0.93 – 2.29%
Weighted average grant-date fair value per share	$14.34	$11.99	$15.61

The following table represents our Stock Plan and Directors Plan restricted stock awards and restricted stock units granted, vested, and forfeited during 2013.



	Number of Shares (in millions)		Weighted Average Grant-Date Fair Value per Share
Nonvested shares at January 1, 2013	14.8		$	33.17
Granted	6.6		42.93
Vested	(4.7	)	32.14
Forfeited	(1.0	)	35.65
Nonvested shares at December 31, 2013	15.7		$	37.43

The following table represents our stock options activity during 2013.



	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, 2013	18.1		$	32.23
Granted	5.4		43.06
Exercised	(4.7	)	27.35
Forfeited/expired	(0.7	)	37.37
Outstanding at December 31, 2013	18.1		$	36.57	7.1	$	256
Exercisable at December 31, 2013	9.0		$	33.48	5.3	$	156

Employee Stock Purchase Plan

The assumptions and resulting fair values were as follows:



	Year Ended December 31
	2013			2012			2011
Expected volatility	27		%	49		%	38		%
Expected dividend yield	1.12		%	1.16		%	0.78		%
Risk-free interest rate	0.06		%	0.11		%	0.14		%
Weighted average grant-date fair value per share	$	8.40		$	8.93		$	11.88