Does Giving an Incentive Really Work on Improving Exam Scores

Short Answer

Does giving an incentive really work on improving exam scores? For example,consider the following treatments to measure the use of incentives.
Treatment 1: No incentive
Treatment 2: Incentive given to all
Treatment 3: Incentive given only if a certain score is received
For this experiment,assume all subjects are seniors at a university.To reduce bias,how should you select the items to use in your study?