In a 2018 study reported in The New England Journal of Medicine, Johnston et al. studied the effect of a combination of the drug clopidogrel and aspirin on reducing the rate of recurrent stroke among stroke patients. Stroke patients in the study were randomly assigned to receive clopidogrel and aspirin (n = 2432) or a placebo and aspirin (n = 2449). Of those receiving clopidogrel and aspirin, 121 had another stroke. Of those receiving the placebo and aspirin, 159 had another stroke. Researchers concluded that patients with minor ischemic stroke or high-risk TIA who received a combination of clopidogrel and aspirin had a lower risk of having another stroke.

a. Compare the percentage in each group who had another stroke. Based on these percentages, does it seem like clopidogrel might be effective in reducing the risk of recurrent stroke?

b. Was this a controlled experiment or an observational study?

c. Identify the treatment and response variables.

d. State the conclusion in terms of cause and effect or explain why cause-and-effect conclusions cannot be drawn from this study.