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A professor was curious about the impact of student diet, especially sugar and caffeine, on test performance. She invited students from her classes to participate in a study before their mid-term exams. Forty students accepted the invitation to participate. The professor randomly assigned them to her experimental groups. The students in one group were told not to change their food or drink consumption during the week before their exams, but to record everything they ate and drank in a diary. The students in the second group were also asked to record their nutritional intake in the same kind of diary, but were also given unlimited free coffee at the campus coffee shop. The students in the third group received both free coffee and free donuts at the coffee shop, and again were asked to record everything they ate and drank. The professor planned to use the average of their mid-term grades as the dependent variable.
1. How would you label the design of this study? (Use words and symbolic notation.)
2. Were the groups equivalent at the onset of the study? How do you know?
3. Are any threats to the internal validity of the study apparent to you? If so, which threats do you see, and how might they affect the study results? Can you think of any alternative designs to address the same research question and increase control of the internal validity threats?