In which scenario would a variable-interval schedule be applied?

A variable-interval schedule involves providing reinforcement after an unpredictable period of time, which makes the behavior produce reinforcements at random intervals. In the case of winning a lottery with random draws, participants do not know when the next reward (winning) will occur; the outcome is based on chance and can happen at varying times. This uncertainty and the potential for a reward at any moment encourage continued participation in the lottery, which aligns perfectly with the concept of a variable-interval schedule.

In contrast, other scenarios mentioned represent different schedules of reinforcement. For instance, receiving a paycheck every two weeks exemplifies a fixed-interval schedule, where the reinforcement (paycheck) is delivered at predictable, regular intervals. Praising a child for cleaning their room reflects a variable ratio schedule if it occurs sporadically but when specific criteria are met; however, it is more characteristically related to fixed or intermittent reinforcement based on behavior. Finally, receiving rewards after every successful project also indicates a fixed ratio schedule, as reinforcement is contingent upon the completion of a specific number of tasks. Thus, the random nature of lottery winnings distinguishes option B as the only scenario that correctly demonstrates a variable-interval schedule.