Diluted earnings per share is calculated by dividing net income by the weighted average number of shares of common stock outstanding, including shares to be issued upon exercise of stock options, performance share awards and restricted stock outstanding under Exelon's LTIPs considered to be common stock equivalents. The following table sets forth the components of basic and diluted earnings per share and shows the effect of these stock options, performance share awards and restricted stock on the weighted average number of shares outstanding (in millions) used in calculating diluted earnings per share:

			Three Months Ended September 30,				Nine Months Ended September 30,
			2012		2011		2012		2011

Net income on common stock		$	296	$	601	$	782	$	1,889

Average common shares outstanding — basic			854		663		804		663
Assumed exercise of stock options, performance share awards
	and restricted stock		3		2		2		1

Average common shares outstanding — diluted			857		665		806		664

The number of stock options not included in the calculation of diluted common shares outstanding due to their antidilutive effect was approximately 18 million and 13 million for the three and nine months ended September 30, 2012, respectively, and 10 million and 9 million for the three and nine months ended September 30, 2011, respectively.

Under share repurchase programs, 35 million shares of common stock are held as treasury stock with a cost of $2.3 billion as of September 30, 2012. In 2008, Exelon management decided to defer indefinitely any share repurchases.