We use data from a promotion campaign of NH-Hoteles to study self-selection of participants in a gift-exchange experiment. The promotion campaign allowed guests to pay any non negative amount of money for a stay in one of 36 hotels in Belgium and the Netherlands. The data allow us to distinguish...

Randomized experiments provide policy relevant treatment effects if there areno spillovers between participants and nonparticipants. We show that thisassumption is violated for a Danish activation program for unemployed workers.Using a difference-in-difference model we show that the...