Cognitive intra-individual variability dispersion as an emerging measure of neuropsychological inference: A narrative review of its history, methodology, empirical support, future directions, and recommendations for best practices

Objective: Cognitive intra-individual variability (IIV) is a commonly used method to estimate how dispersed or inconsistent an examinee’s test scores are across measures that comprise a test battery, trial-by-trial responses on a single task, or across multiple testing sessions. Elevated IIV has been hypothesized to reflect a failure of executive control due to alterations in brain activity or central nervous system integrity. Measures of IIV have a rich history. Growing empirical evidence supports their construct validity and potential for research and clinical applications. However, guidelines for calculating, interpreting, and implementing IIV measures in clinical practice are lacking. Here, we outline the history of IIV and its use in clinical research, summarize metrics for its calculation, review psychometric limitations, and explore future avenues of investigation, with a specific focus on dispersion-based (i.e., variability across tasks) IIV. Methods: A narrative review and commentary on the literature. Conclusions: IIV reliably differentiates clinical populations from healthy groups and predicts disease progression, everyday functioning, and mortality. Questions pertaining to the optimal methodology for IIV, its cognitive architecture, and its incremental validity remain unanswered. We discuss the possibility of using IIV as an embedded measure of performance validity and in detecting pathologic change using ecological momentary assessments. The evidence suggests that IIV can be used as a method of neuropsychological inference. We conclude this review with recommendations for best practices for employing IIV measures in research.