One measure of reinforcer efficacy is choice – if salted caramels are chosen more often than Laffy Taffy, then, for this individual, salted caramels are the more effective of the two reinforcers. But choice does not tell us how much more reinforcing salted caramels are than Laffy Taffy, for this individual. For this we need to measure the maximum amount of behavior the reinforcer will maintain, a measure known as the __