The following table provides a reconciliation between basic and diluted earnings per share for the years ended December 31:

Millions, Except Per Share Amounts	2014		2013		2012
Net income	$	5,180	$	4,388	$	3,943
Weighted-average number of shares outstanding:
Basic		897.1		926.5		946.2
Dilutive effect of stock options		2.1		2.4		3.6
Dilutive effect of retention shares and units		1.9		2.6		3.1
Diluted		901.1		931.5		952.9
Earnings per share – basic	$	5.77	$	4.74	$	4.17
Earnings per share – diluted	$	5.75	$	4.71	$	4.14

Common stock options totaling 0.4 million, 0.5 million, and 1.1 million for 2014, 2013, and 2012, respectively, were excluded from the computation of diluted earnings per share because the exercise prices of these options exceeded the average market price of our common stock for the respective periods, and the effect of their inclusion would be anti-dilutive.