A new study published in the Journal of Affective Disorders, aimed to determine if exercise could effectively treat anxiety. Researchers at the University of Gothenburg studied 258 adults ages 26 to 35 with anxiety syndrome. Patients were randomly assigned to either a moderate exercise group, a strenuous exercise group, or to a group who received advice on physical activity. Anxiety symptoms in all three groups were measured at the beginning and the end of the study by observers who did know who was in which group. Researchers found that both moderate and strenuous exercise groups had a greater reduction in anxiety symptoms compared to the group just getting advice on physical activity. The difference was statistically significant. What is the explanatory variable