A research team conducted an experiment to investigate the effects of different types of music on study performance. In this study, they divided 40 high school students into four groups of 10 students each. Each group was assigned to study in a different environment: one group studied in silence, another group listened to classical music, a third group listened to pop music, and the last group listened to jazz music. After studying, each student took a test to evaluate their retention of the studied material.
Which aspect of experimental design is critical for ensuring the validity of the results obtained from this study?