Mark conducts a study for his research methods class to determine if consuming caffeine causes people to perform better on cognitive tasks. In his study, he gives half of his participants a glass of water and half of his participants a glass of cola and then has them attempt to solve 15 math problems. He finds that people who drink caffeine beforehand solve more math problems than those who drink water beforehand. Mark decides to conduct the exact same study again to ensure that he can find the same difference between the groups a second time. This study is known as a: