Three Months Ended
March 31,

2012

2011

Number of common shares outstanding at end of period

462.7

474.7

Effect of using weighted average common shares outstanding

(0.6

)

1.0

Weighted average basic common shares outstanding

462.1

475.7

Dilutive effect of equity-based compensation awards and other contingently issuable shares

1.3

1.9

Weighted average diluted common shares outstanding

463.4

477.6

Potentially issuable shares

17.4

17.9

Number of anti-dilutive potentially issuable shares excluded from diluted common shares outstanding

8.3

6.2