On April 12, 1955, Dr. Jonas Salk released the results of clinical trials for his vaccine to prevent polio. In these clinical trials, 400,000 children were randomly divided in two groups. The subjects in group 1 (the experimental group) were given the vaccine, while the subjects in group 2 (the control group) were given a placebo. Of the 200,000 children in the experimental group, 33 developed polio. Of the 200,000 children in the control group, 115 developed polio.
(a) What type of experimental design is this?
(b) What is the response variable?
(c) What are the treatments?
(d) What is a placebo?
(e) Why is such a large number of subjects needed for this study?
(f) Does it appear to be the case that the vaccine was effective?

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