Final answer:
A correlated groups design is used to reduce the effects of individual differences among subjects in a study by controlling for variability and isolating the independent variable's effect. The correct answer is option d. to reduce the effects of the individual differences among subjects.
Step-by-step explanation:
One employs a correlated groups design d. to reduce the effects of the individual differences among subjects. This design is applied with the intent of minimizing variability caused by preexisting differences between subjects.
By having participants engage in more than one condition, or by matching participants on key characteristics across conditions, the researcher can control for these individual differences, thereby isolating the effect of the independent variable.
In a correlated groups design, also known as a within-subjects or repeated-measures design, each subject experiences all levels of the independent variable. The control group in such studies serves as a basis for comparison and aids in controlling chance factors by holding them constant across conditions, ensuring the experimental manipulation is the only difference.
By using random assignment to treatments and the inclusion of a control group, the goal is to attribute any observed differences in the dependent variable directly to the independent variable's manipulation, thereby establishing a cause-and-effect relationship.