Earnings Per Share

12 Months Ended

Dec. 31, 2017

15. Earnings Per Share

Basic and diluted earnings per share were computed using the following common share data for the years ended December 31 (shares in millions):


	2017	2016	2015
Number of common shares outstanding at year-end	433.3	439.3	447.2
Effect of using weighted average common shares outstanding	5.5	4.2	5.5
Weighted average basic common shares outstanding	438.8	443.5	452.7
Dilutive effect of equity-based compensation awards and other contingently issuable shares (a)	3.1	3.0	3.2
Weighted average diluted common shares outstanding	441.9	446.5	455.9
Potentially issuable shares	8.1	9.8	10.2
Number of anti-dilutive potentially issuable shares excluded from diluted common shares outstanding	1.9	1.0	2.0

(a)

As of January 1, 2017, we adopted ASU 2016‑09 prospectively and no longer include excess tax benefits as assumed proceeds. See Note 2 for further discussion.