Income per Share
Basic income per share is based on the weighted average number of common shares outstanding during the period. Diluted income per share includes additional common shares that would have been outstanding if potential common shares with a dilutive effect had been issued. Differences between basic and diluted weighted average common shares outstanding for all periods presented resulted from the dilutive effect of awards granted under our stock incentive plans.
Excluded from the computation of diluted income per share are options to purchase three million shares of common stock that were outstanding in 2013, seven million shares of common stock that were outstanding in 2012, and three million shares of common stock that were outstanding in 2011. These options were outstanding during these years but were excluded because they were antidilutive, as the option exercise price was greater than the average market price of the common shares.