Decoy example in the market

Consider the price of drinks at a well-known juice bar: a small (350 ml) size costs $6.10; the medium (450 ml) $7.10; and the large (610 ml) $7.50. The medium is a slightly better value than the small, and the large better still. The medium is designed to be the decoy, steering you to see the biggest drink as the best value for money.

If you buy the biggest, was it because you made a sensible choice, or have you been manipulated to opt for bigger than intended?