This chapter explores within-subjects designs, where the same participants experience all experimental conditions. This approach controls for individual differences, increasing statistical precision and sensitivity to subtle effects. Using the example of museum design, the chapter illustrates how personal responses to varied spaces can be measured without noise from inter-individual variability. It also discusses order effects, counterbalancing techniques, and limitations such as carryover effects when conditions have lasting impacts. The chapter shows how within-subjects designs yield robust causal inferences and are particularly valuable when studying human experience in spaces where personal reactions are critical to design outcomes.