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Earnings per Share
 12 Months Ended
Dec. 31, 2020
Earnings Per Share [Abstract]  
Earnings per Share Earnings per Share
The calculations of earnings per share (shares in millions) are as follows:
Years Ended December 31202020192018
Net income attributable to Merck & Co., Inc.$7,067 $9,843 $6,220 
Average common shares outstanding2,530 2,565 2,664 
Common shares issuable (1)
11 15 15 
Average common shares outstanding assuming dilution2,541 2,580 2,679 
Basic earnings per common share attributable to Merck & Co., Inc. common shareholders
$2.79 $3.84 $2.34 
Earnings per common share assuming dilution attributable to Merck & Co., Inc. common shareholders
$2.78 $3.81 $2.32 
(1)     Issuable primarily under share-based compensation plans.
In 2020, 2019 and 2018, 5 million, 2 million and 6 million, respectively, of common shares issuable under share-based compensation plans were excluded from the computation of earnings per common share assuming dilution because the effect would have been antidilutive.