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:

The Company granted 97 and 163 stock options during the 13 weeks ended May 4, 2019 and May 5, 2018, respectively. The stock-based compensation expense against operating income for stock options was $2,120 and $2,208 for the 13 weeks ended May 4, 2019 and May 5, 2018, respectively. The weighted-average grant date fair value of these stock options was $89.91 and $50.10 for the 13 weeks ended May 4, 2019 and May 5, 2018, respectively. At May 4, 2019, there was approximately $22,944 of unrecognized stock-based compensation expense related to unvested stock options.

The Company issued 39 and 83 restricted stock units during the 13 weeks ended May 4, 2019 and May 5, 2018, respectively. The stock-based compensation expense charged against operating income for restricted stock units was $2,821 and $2,505 for the 13 weeks ended May 4, 2019 and May 5, 2018, respectively. At May 4, 2019, there was approximately $28,810 of unrecognized stock-based compensation expense related to restricted stock units.

The Company issued 21 and 33 performance-based restricted stock units during the 13 weeks ended May 4, 2019 and May 5, 2018, respectively. The stock-based compensation expense charged against operating income for performance-based restricted stock units was $1,711 and $1,457 for the 13 weeks ended May 4, 2019 and May 5, 2018, respectively. At May 4, 2019, there was approximately $15,198 of unrecognized stock-based compensation expense related to performance-based restricted stock units.