Against coefficient of variation for estimation of intraindividual variability with accuracy measures

Previous studies have shown that intraindividual variability (iV) in performance is an important indicator of individual s cognitive functioning and neurological integrity. While most experiments have examined iV of performance using Reaction Time data (RTs), few studies have considered it with accuracy measures (e.g. number or percentage of correct responses). For these two types of measures, intraindividual standard deviation (iSD) or intraindividual coefficient of variation (iCV; intraindividual standard deviation divided by the individual mean) were used as indicators of iV in performance. However, because accuracy data have a lower and an upper bound (in contrast to RTs), we illustrate both formally and with simulated data, that the iCV cannot be used with accuracy measures. We also show that the coefficient iCV is influenced by the number of items which is an issue when dealing with missing data. We further provide formulas that may help researchers to visualize and correctly interpret their data using any spreadsheet software. The current article finally proposes an alternative coefficient (zeta) to examine iV in performance with accuracy measures that shows similar behaviour as does iCV with RTs data.