The first card had eight spots to fill, whilst the other card had ten spots, but the first two came pre-stamped.
Both cards technically had the same terms and value, but in the first case the customer started from scratch, whereas the second card gave the holder a perceived 20 percent head start. The proof was in the result. A few months down the road, the number of customers who earned the free wash with the pre-stamped cards was almost double that of the empty cards.