Up until this point, we have focused exclusively on statistical models of the location (or mean) of a distribution. This chapter focuses on something new, the scale or variability of a distribution. Specifically, this chapter introduces the concept of intra-individual variability (IIV), the variability within individual units across repeated assessments. Although a relatively niche area of study, IIV provides additional information about an individual unit and allows new types of research or practical questions ...