Brian researched how a person's intelligence quotient (IQ) is affected by exercise. First, people took an IQ test. Then, over 2 months, they did different amounts of exercise. Then, Brian gave them a second IQ test.
What is the response variable in this experiment?
Select the correct answer below:
The number of people in the study
The first IQ test score
The amount of exercise the people do
The change in IQ score from the first to second test