pre_text: 11. Stock Award Plan Total recognized stock-based compensation expense is included in our consolidated statements of operations as shown in the table below. 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. The estimated income tax benefit of stock-based compensation expense included in the provision for income taxes for the year ended December 31, 2019 is approximately $5.3 million. No stock-based compensation costs were capitalized during the years ended December 31, 2019, 2018 and 2017. The calculation of stock-based compensation expenses includes an estimate for forfeitures at the time of grant. This estimate can be revised in subsequent periods if actual forfeitures differ from those estimates, which are based on historical trends. Total unrecognized stock-based compensation expense related to nonvested awards and options was $62.4 million as of December 31, 2019, and this expense is expected to be recognized over a weighted-average period of 2.3 years.
table: Year Ended December 31,
table: (In thousands) 2019 2018 2017
table: Cost of revenue:
table: Software delivery, support and maintenance $ 2,075 $ 2,184 $ 2,879
table: Client services 4,067 3,997 4,484
table: Total cost of revenue 6,142 6,181 7,363
table: Selling, general and administrative expenses 27,348 24,213 23,497
table: Research and development 9,200 8,937 8,605
table: Total stock-based compensation expense $ 42,690 $ 39,331 $ 39,465
question: Analyse this data from a financial earnings document. What is the change in the Total cost of revenue from 2018 to 2019?