In 1954, an experiment was designed to test the effectiveness of the Salk vaccine in preventing polio, which had killed or paralyzed thousands of children. By random selection, 401,974 children were assigned to two groups:
Group #1: 200,745 children were given a treatment consisting of a Salk vaccine injection. Among this group, 33 children later developed paralytic polio.
Group #2: 201,229 children were injected with a placebo that contained no drugs. Among this group, 115 children later developed paralytic polio.
Questions:
Is this an observational study or an experimental study, and why?
Between observational and experimental studies, which is a better type of study and why?
What is the percentage of children who developed paralytic polio when injected with the placebo?
What type of sampling did Mr. Salk use, and why do you think so? Please explain your answer in detail.