Stock-based compensation expense recognized during the three months ended March 31, 2019 and 2018 is included in our consolidated statements of operations as shown in the below table. Stock-based compensation expense includes both non-cash expense related to grants of stock-based awards as well as cash expense related to the employee discount applied to purchases of our common stock under our employee stock purchase plan. No stock-based compensation costs were capitalized during the three months ended March 31, 2019 and 2018.

	Three Months Ended March 31,
(In thousands)	2019		2018
Cost of revenue:
Software delivery, support and maintenance	$	513	$	596
Client services		1,093		1,408
Total cost of revenue		1,606		2,004
Selling, general and administrative expenses		8,325		6,132
Research and development		2,877		2,807
Total stock-based compensation expense	$	12,808	$	10,943

We measure stock-based compensation expense at the grant date based on the fair value of the award. We recognize the expense for service-based share awards over the requisite service period on a straight-line basis, net of estimated forfeitures. We recognize the expense for performance-based and market-based share awards over the vesting period under the accelerated attribution method, net of estimated forfeitures. In addition, we recognize stock-based compensation cost for awards with performance conditions if and when we conclude that it is probable that the performance conditions will be achieved.

The fair value of service-based and performance-based restricted stock units is measured at the underlying closing share price of our common stock on the date of grant. The fair value of market-based restricted stock units is measured using the Monte Carlo pricing model. No stock options were granted during the three months ended March 31, 2019 and 2018.

	Three Months Ended March 31, 2019
			Weighted-Average
			Grant Date
(In thousands, except per share amounts)	Shares		Fair Value
Service-based restricted stock units		1,040	$	10.72
Market-based restricted stock units with a service condition		700	$	11.74
		1,740	$	11.13

During the three months ended March 31, 2019 and the year ended December 31, 2018, 1.1 million and 1.6 million shares of common stock, respectively, were issued in connection with the exercise of options and the release of restrictions on stock awards.

Upon vesting, restricted stock units are generally net share-settled to cover the required withholding tax and the remaining amount is converted into an equivalent number of shares of common stock. The majority of restricted stock units and awards that vested during the three months ended March 31, 2019 and 2018 were net-share settled such that we withheld shares with fair value equivalent to the employees’ minimum statutory obligation for the applicable income and other employment taxes, and remitted the cash to the appropriate taxing authorities. Total payments for the employees' minimum statutory tax obligations to the taxing authorities are reflected as a financing activity within the accompanying consolidated statements of cash flows. The total shares withheld for the three months ended March 31, 2019 and 2018 were 508 thousand and 609 thousand, respectively, and were based on the value of the restricted stock units on their vesting date as determined by our closing stock price. These net-share settlements had the effect of share repurchases by us as they reduced the number of shares that would have otherwise been issued as a result of the vesting.

On November 17, 2016, we announced that our Board approved a stock purchase program (the “2016 Program”) under which we may repurchase up to $200 million of our common stock through December 31, 2019. On August 2, 2018, we announced that our Board approved a new stock purchase program (the “2018 Program”) under which we may repurchase up to $250 million of our common stock through December 31, 2020, replacing the 2016 Program. We repurchased 6.1 million shares of our common stock under the 2018 Program for a total of $64.9 million during the three months ended March 31, 2019. The approximate dollar value of shares that may yet be purchased under the 2018 Program is $148.1 million as of March 31, 2019. We repurchased 4.1 million shares of our common stock under the 2016 Program for a total of $57.6 million during the three months ended March 31, 2018. Any future stock repurchase transactions may be made through open market transactions, block trades, privately negotiated transactions (including accelerated share repurchase transactions) or other means, subject to market conditions. Any repurchase activity will depend on many factors such as our working capital needs, cash requirements for investments, debt repayment obligations, economic and market conditions at the time, including the price of our common stock, and other factors that we consider relevant. Our stock repurchase program may be accelerated, suspended, delayed or discontinued at any time.