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Earnings Per Share
 12 Months Ended
Dec. 31, 2020
Earnings Per Share  
Earnings Per Share

16.  Earnings Per Share

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

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2020

    

2019

    

2018

Number of common shares outstanding at end of period

 

422.8

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424.3

 

424.0

Effect of using weighted average common shares outstanding

 

0.2

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0.3

 

5.1

Weighted average basic common shares outstanding

 

423.0

​

424.6

 

429.1

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

 

2.1

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2.9

 

3.1

Weighted average diluted common shares outstanding

 

425.1

​

427.5

 

432.2

Potentially issuable shares

 

6.1

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6.7

 

7.4

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

 

1.6

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0.7

 

1.5

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