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