A researcher does a study examining the effects of a preschool program. He uses a nonequivalent comparison-group design. He finds that the cognitive growth of his experimental group is greater than that of his control. Unfortunately, he later finds that in general children who live in the area where he drew his experimental group tend to grow faster cognitively than children who were from the area where he drew his control group. When he discovered this problem, he discovered what threat to the internal validity of his study?
a. Selection-maturation effect
b. History effect
c. Selection-instrumentation effect
d. Testing effect
Ans: a
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