Question: Identify each of the following studies as a survey, an experiment, an observational study, or a case study. Explain your reasoning.
a. A doctor claims to be able to cure migraine headaches. A researcher administers a questionnaire to each of the patients the doctor claims to have cured.
b. Patients who visit a clinic to help them stop smoking are given a choice of two treatments: undergoing hypnosis or applying nicotine patches. The percentages who quit are compared for the two methods.
c. A large company wants to compare two incentive plans for increasing sales. The company randomly assigns a number of its sales staff to receive each kind of incentive and compares the average change in sales of the employees under the two plans.