19. Net Income per Common Share

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Basic income per common share is calculated based on the weighted average number of shares of common stock outstanding for the period. Diluted income per common share is calculated based on the assumption that stock options outstanding, which have an exercise price less than the average market price of the Company’s common stock during the period, would have been exercised on the later of the beginning of the period or the date granted and that the funds obtained from the exercise were used to purchase common shares at the average market price during the period. All the Company’s restricted stock units are considered to be dilutive.

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The effect of the Company’s dilutive securities is calculated using the treasury stock method and only those instruments that result in a reduction in net income per common share are included in the calculation. Options to purchase 2.2 million, 4.2 million, and 3.6 million shares of common stock at weighted average exercise prices of $10.69, $8.95, and $10.44 were outstanding as of December 31, 2021, 2020, and 2019, respectively, but were not included in the computation of diluted weighted average common shares outstanding, as the exercise price of the options exceeded the average price of the Company’s common stock during those periods, and therefore were anti-dilutive.

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Basic and diluted net income per common share is calculated as follows:

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