Earnings per Share

Basic earnings per share is calculated by dividing net income by the weighted average number of shares outstanding during the reported period. The calculation of diluted earnings per share is similar to basic earnings per share, except that the weighted average number of shares outstanding includes the dilution from potential shares resulting from stock options, restricted stock awards and other stock awards. The components of basic and diluted earnings per share are as follows:



	Three Months Ended September 30,				Nine Months Ended September 30,
In millions, except per share amounts	2018		2017		2018		2017
Net income attributable to common stockholders	$	18	$	13	$	15	$	7
Weighted average outstanding shares of common stock	118.7		123.7		119.9		127.3
Dilutive effect of employee stock options, restricted stock and other stock awards	2.0		2.1		1.9		1.8
Common stock and common stock equivalents	120.7		125.8		121.8		129.1
Net income per share:
Basic	$	0.15	$	0.11	$	0.13	$	0.05
Diluted	$	0.15	$	0.10	$	0.12	$	0.05

Options to purchase 2.5 million shares of common stock for the three and nine months ended September 30, 2018 and 2.7 million and 3.6 million shares of common stock for the three and nine months ended September 30, 2017 were not included in the computation of diluted earnings per share. The exercise prices of these options were greater than the average market price of the common shares for the period, and therefore would have been anti-dilutive.