The following table shows the calculation of basic and diluted net income per share attributable to Gilead common stockholders (in millions except per share amounts):

The denominator for diluted earnings per share was increased by 13 million shares for the effect of options, while Gilead had more than 19 million options exercisable at 12/31/2016.

What are factors that could explain the difference?

What does it mean for options to be anti-dilutive?

Year Ended December 31, 2015_ 2016 2014 Net income attributable to Gilead $13,501 $ 18,108 $ 12,101 Shares used in per share calculation - basic Effect of dilutive securities: 1,339 1,464 1,522 Stock options and equivalents 13 23 Conversion spread related to the Convertible Notes 2 14 30 Warrants related to the Convertible Notes 4 20 62 Shares used in per share calculation - diluted 1,358 1,521 1,647 Net income per share attributable to Gilead common stockholders - basic $ 10.08 $12.37 $ 7.95 11.91 5 7.35 Net income per share attributable to Gilead common stockholders - diluted $ 9.94$11.91 $7.35