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Earnings Per Share
 12 Months Ended
Dec. 31, 2018
Earnings Per Share  
Earnings Per Share

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):

 

 

 

 

 

 

 

 

    

2018

    

2017

    

2016

Number of common shares outstanding at end of period

 

424.0

 

433.3

 

439.3

Effect of using weighted average common shares outstanding

 

5.1

 

5.5

 

4.2

Weighted average basic common shares outstanding

 

429.1

 

438.8

 

443.5

Dilutive effect of equity-based compensation awards and other contingently issuable shares (a)

 

3.1

 

3.1

 

3.0

Weighted average diluted common shares outstanding

 

432.2

 

441.9

 

446.5

Potentially issuable shares

 

7.4

 

8.1

 

9.8

Number of anti-dilutive potentially issuable shares excluded from diluted common shares outstanding

 

1.5

 

1.9

 

1.0

(a)As of January 1, 2017, we adopted ASU 2016-09 prospectively and no longer include excess tax benefits as assumed proceeds.