 The roles of responses were reversed in the second test, with corresponding results.That is, only children who preferred playing pinball over eating candy showed a reinforcement effect.This study, among others, helps to confirm the Premack principle in showing that a high-probability activity can be an effective reinforcer for an activity that the subject is less likely to perform 

Premack's principle may be violated if a situation or schedule of reinforcement provides much more of the high-probability behavior than of the low-probability behavior.