This chapter examines how to define, manipulate, and control variables to ensure experimental validity. It distinguishes independent, dependent, and confounding variables, emphasizing the need to isolate causal factors. Architectural examples illustrate how uncontrolled variables—like prior familiarity with a space—can distort findings if not managed. Strategies such as randomization, blinding, and control conditions are discussed. The chapter also explores ecological validity, balancing methodological rigor with real-world relevance. By mastering variable design, researchers can design experiments that yield clear, defensible conclusions about how specific architectural elements influence behavior and perception.