Refer to Exercise 50 for a description of a study about using calcium to lower systolic blood pressure.
Exercise 50
Researchers wanted to determine whether calcium can lower the systolic blood pressure of African American men. They randomly assigned 21 men to two groups. Each member of one group took a calcium supplement for 12 weeks, and each member of the other group took a placebo during the same period. The systolic blood pressures (in mmHg) of the men before and after the 12-week period are shown in Table 19.

a. Is the study an experiment or an observational study? Explain.
b. What are the explanatory and response variables?
c. What does random assignment mean in this study? How could this be accomplished?
d. Describe the sample and the population.
e. After performing a statistical test, the study concluded that calcium lowers systolic blood pressure in African American men. Does this mean there is causality or only an association between the two variables? Explain.

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Table 19 Levels of Systolic Blood Pressure in 21 African American Men Systolic Blood Pressure (mmHg) Placebo Group Calcium Group Before After Difference Before AfterDifference 124 97 113 105 95 123 109 100 114 105 112 115 116 106 102 125 104 107 110 123 102 98 114 119 114 114 121 118 107 110 136 102 130