Basic net income per common share is calculated based on the weighted average number of common shares outstanding during the period. Diluted net income per common share is computed by dividing net income by the weighted average number of common and potentially dilutive common shares outstanding during the period. Potentially dilutive common shares of our stock result from dilutive common stock options and restricted stock units. We use the treasury stock method to calculate the weighted-average shares used in the diluted earnings per share computation. Under the treasury stock method, the proceeds from exercise of an option, the amount of compensation cost, if any, for future service that we have yet recognized, and the amount of estimated tax benefits that would be recorded in paid-in capital, if any, when the option is exercised are assumed to be used to repurchase shares in the current period.

 

The number of shares used to calculate earnings per share are as follows (in thousands, except per share data):

 

 

The dilutive effect of stock options in the above table excludes all options for which the aggregate exercise proceeds exceeded the average market price for the period. The number of potentially dilutive option shares excluded from the calculation was

million, million, and at and respectively.