28. Warrant plans

Presented below is a summary of warrant activities for the reported periods. Various warrant plans were approved for the benefit of our employees, and for directors and independent consultants of Galapagos NV. For warrant plans issued prior to 2011, the warrants offered to the employees and independent consultants vest according to the following schedule: 10% of the warrants vest on the date of the grant; an additional 10% vest at the first anniversary of the grant; an additional 20% vest at the second anniversary of the grant; an additional 20% vest at the third anniversary of the grant; and an additional 40% vest at the end of the third calendar year following the grant.

The warrants granted under warrant plans created from 2011 onwards vest at the end of the third calendar year following the year of the grant, with no intermediate vesting, with the exception of the warrants granted under Warrant Plan 2015 (B), Warrant Plan 2015 RMV, and Warrant Plan 2016 (B), which vest on the third anniversary of the notary deed enacting the acceptance and issuance of the warrants.

The warrants offered to directors vest over a period of 36 months at a rate of 1/36th per month.

Warrants cannot be exercised before the end of the third calendar year following the year of the grant, except for warrants granted under Warrant Plan 2015 (B), Warrant Plan 2015 RMV, and Warrant Plan 2016 (B), which become exercisable on the third anniversary of the notary deed enacting the acceptance and issuance of the warrants. In the event of a change of control over Galapagos NV, all outstanding warrants vest immediately and will be immediately exercisable.

The table below sets forth a summary of warrants outstanding and exercisable at 31 December 2019, per warrant plan:

The table below sets forth the inputs into the valuation of the warrants.

Warrant plans

The exercise price of the warrants is determined pursuant to the applicable provisions of the Belgian Companies Code.

The weighted average estimated volatility is calculated on the basis of the implied volatility of the share price over the expected life of the warrants.

The weighted average expected life of the warrant is calculated as the estimated duration until exercise, taking into account the specific features of the plans.

Our share based compensation expense in 2019 amounted to €38,297 thousand (2018: €26,757 thousand).

The following table provides an overview of the outstanding warrants per category of warrant holders at 31 December 2019 and 31 December 2018.

The outstanding warrants at the end of the accounting period have an average exercise price of €70.09 (2018: €53.30) and a weighted average remaining expected life of 1,439 days (2018: 1,500 days).