NOTE 13. EARNINGS PER SHARE

Basic earnings per share is computed by dividing net income attributable to IDEXX Laboratories, Inc. stockholders by the weighted average number of shares of common stock and vested deferred stock units outstanding during the year. The computation of diluted earnings per share is similar to the computation of basic earnings per share, except that the denominator is increased for the assumed exercise of dilutive options and assumed issuance of unvested restricted stock units and unvested deferred stock units using the treasury stock method unless the effect is anti-dilutive. The treasury stock method assumes that proceeds, including cash received from the exercise of employee stock options, the total unrecognized compensation expense for unvested share-based compensation awards and, prior to the adoption of new accounting guidance related to share-based compensation on January 1, 2017, the tax benefits resulting from share-based compensation tax deductions in excess of the related expense recognized for financial reporting purposes, would be used to purchase our common stock at the average market price during the period. For further discussion regarding the impact of the new accounting guidance related to share-based compensation, see Note 2. Vested deferred stock units outstanding are included in shares outstanding for basic and diluted earnings per share because the associated shares of our common stock are issuable for no cash consideration, the number of shares of our common stock to be issued is fixed and issuance is not contingent. See Note 4 for additional information regarding deferred stock units.

The following is a reconciliation of shares outstanding for basic and diluted earnings per share for the years ended December 31, 2017, 2016 and 2015 (in thousands):


	For the Years Ended December 31,
	2017	2016	2015

Shares outstanding for basic earnings per share:	87,769	89,732	92,601

Shares outstanding for diluted earnings per share:
Shares outstanding for basic earnings per share	87,769	89,732	92,601
Dilutive effect of share-based payment awards	1,798	1,152	1,048
	89,567	90,884	93,649

Certain options to acquire shares have been excluded from the calculation of shares outstanding for dilutive earnings per share because they were anti-dilutive. The following table presents information concerning those anti-dilutive options for the years ended December 31, 2017, 2016 and 2015 (in thousands):

For the Years Ended December 31,

Weighted average number of shares underlying anti-dilutive options