Basic net income per common share is based on the weighted average number of common shares outstanding. Diluted net income per common share is based on the weighted average number of common shares outstanding and potentially dilutive shares. The dilutive effect of employee stock-based compensation is included in the computation of diluted net income per common share. The dilutive effect of stock-based compensation is calculated using the treasury stock method and expected proceeds upon exercise or issuance of the stock-based compensation. The following table summarizes the shares, in thousands, used in computing basic and diluted net income per common share:


	Six MonthsEnded		ThreeMonths Ended
	June 30,		June 30,
	2011	2010	2011	2010
Average common shares outstanding during
the period for basic computation	137,971	136,647	138,114	136,785
Dilutiveeffect of employeestock-based compensation	547	313	667	227
Average common shares outstanding during
the period for diluted computation	138,518	136,960	138,781	137,012

For the six and three months ended June 30, 2011, employee stock options to purchase 933,800 shares of common stock, were excluded from the calculations of diluted net income per share as the calculated proceeds from the options' exercise were greater than the average market price of the Company's common stock during these periods. For the six and three months ended June 30, 2010, employee stock options to purchase 2,665,445 shares of common stock, were excluded from the calculations of diluted net income per share as the calculated proceeds from the options' exercise were greater than the average market price of the Company's common stock during these periods.