pre_text: Stock-based Compensation Expense The following table sets forth our stock-based compensation expense for the years ended December 31, 2019, 2018 and 2017 (in thousands): Stock-based compensation awards include employee stock options, restricted stock awards and units, and employee stock purchases. For the year ended December 31, 2019, stock-based compensation expense was $31.6 million, of which $0.2 million related to employee stock options, $29.1 million related to restricted stock awards and units and $2.3 million related to employee stock purchases. For the year ended December 31, 2018, stock-based compensation expense was $31.0 million, of which $0.4 million related to employee stock options, $28.0 million related to restricted stock awards and units and $2.6 million related to employee stock purchases. The increase in stock-based compensation expense in 2019 compared to 2018 was due primarily to a higher volume of restricted stock unit grants.
table: Years Ended December 31,
table: 2019 2018 2017
table: Research, development and other related costs $14,643 $13,168 $13,277
table: Selling, general and administrative 16,911 17,843 20,185
table: Total stock-based compensation expense $31,554 $31,011 $33,462
question: Analyse this data from a financial earnings document. What is the overall proportion of employee stock options and employee stock purchases over total stock-based compensation expense in 2019?