21. Basic and diluted (loss) earnings per share

Basic (loss) earnings per share is calculated by dividing net (loss) income attributable to Atlantic Power Corporation by the weighted average common shares outstanding during their respective periods. Shares issued and shares repurchased during the year are weighted for the portion of the year that they were outstanding. Diluted (loss) earnings per share is computed in a manner consistent with that of basic (loss) earnings per share while giving effect to all potentially dilutive common shares that were outstanding during the period. The dilutive effect of our convertible debentures is calculated using the “if-converted method.” Under the if-converted method, the debentures are assumed to be converted at the beginning of the period, and the resulting common shares are included in the denominator of the diluted (loss) earnings per share calculation for the entire period being presented. Interest expense, net of any income tax effects, would be added back to the numerator for purposes of the if-converted calculation. The outstanding equity compensation for non-vested LTIP and Transition Equity Participation Agreement notional shares are not considered outstanding for purposes of computing basic (loss) earnings per share. However, these instruments are included in the denominator, when dilutive, for purposes of computing diluted (loss) earnings per share under the treasury stock method.

The following table sets forth the diluted net (loss) income and potentially dilutive shares utilized in the per share calculation for the years ended December 31, 2019, 2018 and 2017:


Basic	2019		2018		2017
Numerator:
(Loss) income attributable to Atlantic Power Corporation	$	(42.6)	$	36.8	$	(98.6)
Denominator:
Weighted average basic shares outstanding		109.3		112.0		115.1
Basic (loss) earnings per share attributable to Atlantic Power Corporation	$	(0.39)	$	0.33	$	(0.86)
Diluted
Numerator:
Net (loss) income attributable to Atlantic Power Corporation		(42.6)		36.8		(98.6)
Add: convertible debenture interest expense		—		4.7		—
		(42.6)		41.5		(98.6)
Denominator:
Weighted average basic shares outstanding		109.3		112.0		115.1
Convertible debentures		—		27.8		—
Share-based compensation		—		2.0		—
		109.3		141.8		115.1
Diluted (loss) earnings per share attributable to Atlantic Power Corporation		(0.39)		0.29		(0.86)

The following table summarizes our outstanding instruments that are anti-dilutive and were not included in the computation of our diluted (loss) `earnings per share:


	2019		2018		2017
Share-based compensation		1.5		—		1.6
Convertible debentures		27.8		—		8.1
Total		29.3		—		9.7