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A scientist is interested determining whether folic acid supplementation affects the development of heart disease, measured by changes in blood pressure. Study participants are divided into two groups. Individuals in one group take a folic acid supplement each day, and those in the other group are given a placebo. Participants are instructed to eat and exercise as normal. The scientist measures blood pressure at the start of the study and six months later. She finds that participants who took folic acid supplements lowered their blood pressure by 10 units more on average than those on the placebo. In the above study, exercise represents which of the following variables?

A Controlled variable

This question asks what type of variable exercise is in the study described. All study participants were instructed to eat and exercise as normal. Any variable held constant between the experimental and control group is a controlled variable. Since exercise is the same in both groups, it is a controlled variable. The independent variable is the factor that is changed between the two groups; in this experiment, this is folic acid supplementation. The dependent variable is the quantity that is measured—blood pressure in this experiment. The control group is not a variable but is a group of subjects that is identical to the experimental group except for the one dependent variable that is being studied. In this study, the people receiving the placebo are the control group.