Apharmaceutical company has developed an experimental drug meant to relieve symptoms associated with the common cold. the company identifies 300 adult males 25 to 29 years old who have a common cold and randomly divides them into two groups. group 1 is given the experimental drug, while group 2 is given a placebo. after 1 week of treatment, the proportions of each group that still have cold symptoms are compared. this is an example of