In a recent study, researchers found that students who participated in daily physical exercise scored an average of 15% higher on math tests than those who did not exercise regularly. Supporters of daily physical exercise argue that encouraging students to engage in physical activity will not only improve their overall health but also significantly boost their academic performance.
However, some critics claim that the results of this study do not conclusively demonstrate a direct connection between exercise and academic performance since other factors, such as nutrition, study habits, and parental involvement, were not controlled for in the research.
Which of the following, if true, most strengthens the supporters' argument against the critics?