11.Stock-based compensation

The Company measures stock-based compensation expense on the grant date, based on the fair value of the award, and recognizes the expense on a straight-line basis over the requisite service period for awards expected to vest. The Company estimated the grant date fair value of stock options using a Black-Scholes valuation model using the following weighted-average assumptions for the periods indicated:

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The Company granted 97 and 163 stock options during the 26 weeks ended August 3, 2019 and August 4, 2018, respectively. The stock-based compensation expense charged against operating income for stock options was $2,199 and $2,215 for the 13 weeks ended August 3, 2019 and August 4, 2018, respectively. The stock-based compensation expense charged against operating income for stock options was $4,319 and $4,423 for the 26 weeks ended August 3, 2019 and August 4, 2018, respectively. The weighted-average grant date fair value of these stock options was $89.91 and $50.10 for the 26 weeks ended August 3, 2019 and August 4, 2018, respectively. At August 3, 2019, there was approximately $20,473 of unrecognized stock-based compensation expense related to unvested stock options.

The Company issued 47 and 92 restricted stock units during the 26 weeks ended August 3, 2019 and August 4, 2018, respectively. The stock-based compensation expense charged against operating income for restricted stock units was

$3,422 and $3,290 for the 13 weeks ended August 3, 2019 and August 4, 2018, respectively. The stock-based compensation expense charged against operating income for restricted stock units was $6,243 and $5,795 for the 26 weeks ended August 3, 2019 and August 4, 2018, respectively. At August 3, 2019, there was approximately $27,170 of unrecognized stock-based compensation expense related to restricted stock units.

The Company issued 21 and 33 performance-based restricted stock units during the 26 weeks ended August 3, 2019 and August 4, 2018, respectively. The stock-based compensation expense charged against operating income for performance-based restricted stock units was $1,390 and $1,722 for the 13 weeks ended August 3, 2019 and August 4, 2018, respectively. The stock-based compensation expense charged against operating income for performance-based restricted stock units was $3,101 and $3,179 for the 26 weeks ended August 3, 2019 and August 4, 2018, respectively. At August 3, 2019, there was approximately $12,745 of unrecognized stock-based compensation expense related to performance-based restricted stock units.