Researchers handed out loyalty cards at a car wash. The cards offered a stamp for every car wash the customer bought.
Half of the cards had spots for eight stamps, with a free car wash offered for collecting all eight. The other half had spots for ten stamps, but two of the spots came pre-stamped (oooooooo vs ooooooooXX).
The pre-stamped cards resulted in 178% more repeat business than the unstamped ones. No specific tool for that