Question 5.16
5.16 Do antioxidants prevent cancer? 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 clinical trial studied this question with 864 people who were at risk for colon cancer. The subjects were divided into four groups: daily beta-carotene, daily vitamins C and E, all three vitamins every day, and daily placebo. After four years, the researchers were surprised to find no significant difference in colon cancer among the groups.
(a) What are the explanatory and response variables in this experiment?
(b) Outline the design of the experiment. (The diagrams in Figures 5.2 and 5.3 are models.)
(c) Assign labels to the 864 subjects. If you have access to statistical software, use it to choose the first five subjects for the beta-carotene group. Otherwise, use Table A, starting at line 118, to choose the first five subjects for the beta-carotene group.
(d) What does “no significant difference” mean in describing the outcome of the study?
(e) Suggest some lurking variables that could explain why people who eat lots of fruits and vegetables have lower rates of colon cancer. The results of the experiment suggest that these variables, rather than the antioxidants, may be responsible for the observed benefits of fruits and vegetables.