Basic net income per common share is based on the weighted average number of common shares outstanding and the minimum number of shares to be issued upon settlement of the stock purchase contracts issued under the tangible equity units. Diluted net income per common share is based on the weighted average number of common shares outstanding, potentially dilutive shares, and the expected number of shares to be issued upon settlement of the stock purchase contracts issued under the tangible equity units, based on the applicable market value of our common stock. 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 treasury stock method assumes that the proceeds from stock-based compensation are used to purchase the Company’s common stock at the average market price during the period. The following table summarizes the shares, in thousands, used in computing basic and diluted net income per common share:



	Three Months Ended		Nine Months Ended
	September 30,		September 30,
	2019	2018	2019	2018
Average common shares outstanding during the period for basic computation	232,053	177,923	209,971	177,876
Dilutive effect of tangible equity units	-	-	-	-
Dilutive effect of employee stock-based compensation	411	434	364	471
Average common shares outstanding during the period for diluted computation	232,464	178,357	210,335	178,347

For the three months ended September 30, 2019 and 2018 and the nine months ended September 30, 2019, all of the Company’s employee stock options were included in the calculations of diluted net income per share as the calculated cost to exercise the stock options was less than the average market price of the Company’s common stock during these periods. For the nine months ended September 30, 2018, employee stock options to purchase 8,596 shares of common stock were excluded from the calculation of diluted net income per share as the calculated cost to exercise the stock options was greater than the average market price of the Company’s common stock during this period.