Researchers Wilt et al. (New England Journal of Medicine, 2012) investigated whether surgery, compared to just observation, was (more) effective in improving men’s survival chances aft er being diagnosed with prostate cancer. Th e researchers identified 731 men with localized prostate cancer who volunteered to participate. They randomly assigned 364 men to surgery and the remaining 367 to observation. All participants were followed for about 10 years. In those 10 years, 21 surgery recipients died of prostate cancer related reasons compared to 31 observation recipients.
a. Identify the observational units. Choose one. A. Men
B. Men with prostate cancer
C. Men with prostate cancer who underwent surgery
D. Men with prostate cancer who received observation

b. What type of study is this? Choose one.
A. Experiment
B. Observational study
c. What is the primary purpose of random assignment in this type of study? Choose one.
A. To ensure that subjects are representative of the population of interest
B. To ensure that the groups are of equal sizes
C. To create treatment groups that are alike in all aspects except for the treatment administered
D. To improve accuracy of results

d. Identify the explanatory variable. Choose one.
A. Whether or not man dies of prostate cancer related reasons
B. Whether or not man undergoes surgery
C. Percentage of men who die of prostate cancer related reasons
D. The number of men who undergo surgery and the number of men who are just observed

e. Identify the response variable. Choose one.

A. Whether or not man dies of prostate cancer related reasons
B. Whether or not man undergoes surgery
C. Percentage of men who die of prostate cancer related reasons
D. Th e number of men who undergo surgery, and the number of men who are just observed

f. The explanatory variable is ________ (categorical/ quantitative).
g. Th e response variable is ________ (categorical/ quantitative).
h. In this context, which of the following is an (are) appropriate statistic value(s)? Choose all that apply.
A. 21/731 − 31/731 = −0.014
B. 21 − 31 = −10
C. 21/364 − 31/367 = −0.027
D. (31/367)/(21/364) = 1.46

i. Give two possible explanations for the observed statistic value(s) as selected in part (h).