Weighted Average Shares

12 Months Ended

Dec. 31, 2017

Weighted-average shares were calculated as follows (in thousands):



	Year Ended December 31,
	2017	2016	2015
Basic weighted-average common shares outstanding	173,287	170,676	172,592
Effect of dilutive stock options	6,264	3,468	3,390
Diluted weighted-average common and common-equivalent shares outstanding	179,551	174,144	175,982

Stock options to purchase 3,363,141, 4,391,598, and 6,070,156 shares of common stock, on a weighted-average basis, were outstanding in 2017, 2016, and 2015, respectively, but were not included in the calculation of dilutive net income per share because they were anti-dilutive.