weighted average number of common shares and potential dilutive securities 155041 149614 149393 ( 1 ) excludes ( in thousands of shares ) 7731 , 780 and 719 of anti-dilutive shares for the years ended december 31 , 2008 , 2007 and 2006 , respectively . also excludes the 3.75% ( 3.75 % ) exchangeable senior notes due november 2011 ( 201cexchangeable notes 201d ) issued in 2006 , that have an anti-dilutive effect on earnings per share for the years ended december 31 , 2008 , 2007 and 2006