Question: Observational studies had suggested that vitamin E reduces the risk of heart attack. Careful experiments however showed that vitamin E. has no effect at least for women. According to a commentary in the Journal of the American Medical Association.

Thus vitamin E enters the category of the therapies that were promising in epidemiological and observational studies but failed to deliver in adequately powered randomized controlled trails. As in other studies the health user biases must be considered ie the healthy life styles behaviors that characterize individuals who care enough about their health to take various supplements are actually responsible for the better health but this is minimized what the rigorous trail design.

A friend who know s no statistics as you to explain this:

A) What is the difference between observational studies and experiments?

B) What is a randomized controlled trial?

C) How does healthy use bias explain how people who take vitamin E supplements have a better health in observational studies but not in controlled experiments?