Gerontologists interested in the effects of aging on reaction time have two groups of subjects take a test in which they must indicate as quickly as possible whether a probe word is a member of a previous set of words. One group of subjects is between the ages of 25 and 45, whereas the other group is between the ages of 55 and 75. The time it takes to make the response is measured. In this study:
a. What is the independent variable?
b. What is the dependent variable?
c. Identify the control and experimental groups.
d. Is the independent variable manipulated, or is it a participant variable?