Which of the following statistical studies is an observational study?
A. A group of students a survey to determine the percentage who believe in reincarnation
B. a random sample students is shown a documentary on the evolution of reincarnation beliefs throughout human history. A second random sample is not shown the documentary. The object of the study is to determine whether viewing the documentary affects believe in reincarnation.
C. To determine whether vitamin C has any effectiveness at lessening the duration of the common cold, one set of cold sufferers is given no vitamin C, a second set is given 2 grams per day, and a third set is given five grams per day.
D. All of the studies are observational.
E. None of the studies are observational.
Answer: A