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 /
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.