8.28 Observation versus experiment. Observational stud ies had suggested that vitamin E reduces the risk of heart dis ease. Careful experiments, however, showed that vitamin E has no effect. According to a commentary in the Journal of the American Medical Association:

Thus, vitamin E enters the category of therapies that were promising in epidemiologic and observational studies but failed to deliver in adequately powered randomized controlled trials. As in other studies, the

"healthy Mser" bias must be considered, ie, the healthy lifestyle behaviors that characterize individuals who care enough about their health to take various supple ments are actually responsible for the better health, but this is minimized with the rigorous trial design."^

A friend who Tnows no statistics asks you to explain this.

(a) What is the difference between observational studies and experiments?

(b) What is "randomized controlled trial"?

(c) How does "healthy user bias"explain how people who take vitamin E supplements have better health in observational studies but ndt in controlled experiments?