People who eat lots of fruits and vegetables have lower rates of colon cancer than those who eat little of

these foods. Fruits and vegetables are rich in "antioxidants" such as vitamins A, C, and E. Will taking

antioxidants help prevent colon cancer? A medical experiment studied this question with 864 people

who were at risk of colon cancer. The subjects were divided into four equal groups (equal group sizes

are convenient, but not necessary) by assigning following vitamins: daily beta-carotene, daily vitamins

C and E, all three vitamins every day, or daily placebo. After four years, the researchers were surprised

to fifind no signifificant difffference in colon cancer among the groups.

(a) Identify experimental units in this study.

(b) What are the factor(s) and response variable(s) in this experiment?

(c) List all the treatments in this experiment.

(d) Use a diagram to outline the design of the experiment.

(e) The study was double-blind. What does this mean?

(f) What does "no signifificant difffference" mean in describing the outcome of the study?

(g) Suggest a lurking variable that could explain why people who eat lots of fruits and vegetables

have lower rates of colon cancer. The experiment suggests that these variables, rather than the

antioxidants, may be responsible for the observed benefifits of fruits and vegetables.