ABSTRACT
The idea that intraindividual variability (IIV) is a meaningful construct with important clinical implications for the medical 1 and rehabilitation 2 sciences is a relatively old one. Research on the impact of IIV on measurement reliability, risk prediction, and clinical outcomes can be traced to the late 1980s and early 1990s (e.g., Slaughter, Goldberg, Choi, Adams, & Middleton, 1987). However, despite early demonstrations of its significance, broader acceptance has taken time to develop. Early conceptualizations of many illnesses and rehabilitation issues as states rather than processes, and concerns about patient burden related to the use of intensive repeated-measures designs, slowed the adoption of IIV methodologies. Over time, however, more and more researchers began to pay attention to discrepancies in interindividual versus intraindividual results. They began to recognize the potential of intraindividual methodologies to provide unique understanding of illness and recovery as processes with trajectories that can vary by patient and that are influenced by time-varying associations with comorbid conditions and symptoms. With the development of newer technologies that reduced patient burden (e.g., ambulatory monitoring, smartphones) and the introduction of statistical techniques (e.g., multilevel modeling [MLM]) to these fields, the questions being addressed have become increasingly sophisticated, going beyond the quantification of fluctuation within individual patients to the investigation of patterns of symptom covariation in comorbid conditions (e.g., sleep and pain) and the examination of variability as a treatment target in its own right. This chapter is not intended as an exhaustive review; instead, its goal is to provide prototypical examples of key developments in the use of IIV methodologies in the medical and rehabilitation sciences. Although some attention is given to life-span issues, more research in these areas has focused on adulthood (across all developmental stages) with relatively less attention given to earlier periods of the life span.
