10.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 248 and 97 stock options during the 13 weeks ended May 2, 2020 and May 4, 2019, respectively. The stock-based compensation expense against operating income for stock options was $2,475 and $2,120 for the 13 weeks ended May 2, 2020 and May 4, 2019, respectively. The weighted-average grant date fair value of these stock options was $54.40 and $89.91 for the 13 weeks ended May 2, 2020 and May 4, 2019, respectively. At May 2, 2020, there was approximately $26,649 of unrecognized stock-based compensation expense related to unvested stock options.

The Company issued 152 and 39 restricted stock units during the 13 weeks ended May 2, 2020 and May 4, 2019, respectively. The stock-based compensation expense charged against operating income for restricted stock units was $4,187 and $2,821 for the 13 weeks ended May 2, 2020 and May 4, 2019, respectively. At May 2, 2020, there was approximately $42,550 of unrecognized stock-based compensation expense related to restricted stock units.

The Company issued 21 performance-based restricted stock units during the 13 weeks ended May 4, 2019. The Company did not issue any performance-based restricted stock units during the 13 weeks ended May 2, 2020. The stock-based compensation benefit included in operating income for performance-based restricted stock units was $480 for the 13 weeks ended May 2, 2020. The stock-based compensation expense charged against operating income for performance-based restricted stock units was and $1,711 for the 13 weeks ended May 4, 2019. At May 2, 2020, there was approximately $2,123 of unrecognized stock-based compensation expense related to performance-based restricted stock units.