Income per Share

12 Months Ended

Dec. 31, 2011

Income per Share [Abstract]

Income per Share

Note 11. 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.

A reconciliation of the number of shares used for the basic and diluted income per share calculations is as follows:

Millions of shares	2011		2010		2009
Basic weighted average common shares outstanding		918		908		900
Dilutive effect of stock options		4		3		2
Diluted weighted average common shares outstanding		922		911		902

Excluded from the computation of diluted income per share are options to purchase three million shares of common stock that were outstanding in 2011, five million shares of common stock that were outstanding in 2010, and seven million shares of common stock that were outstanding in 2009. 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.