Question: Suppose that a statistics teacher wants to know whether the number of hours students spend studying in a group affects the final course grade. In each part, explain whether the research method described is a randomized experiment or an observational study.

a. Each student keeps a log of the hours he or she spends studying in a group and reports the total after the course is completed.

b. Students are randomly assigned to study groups. The teacher tells each group how often to meet. This varies from 1 hour the day before each exam to 2 hours per week.

c. Students voluntarily join groups based on how often the groups will meet. The groups are designated as meeting weekly, meeting only before exams, or meeting whenever enough members feel that it is necessary.