Investigating Reliabilities of Intraindividual Variability Indicators

Reliabilities of the two most widely used intraindividual variability indicators, ISD ² and ISD, are derived analytically. Both are functions of the sizes of the first and second moments of true intraindividual variability, the size of the measurement error variance, and the number of assessments within a burst. For comparison, the reliability of the intraindividual mean, X̄, is also derived. Simulations are conducted to confirm accuracy of the derived functions. The influences of different factors on the reliabilities of these indicators are also studied. Results show that the reliabilities of ISD ² and ISD are lower than the reliability of X̄ in all studied conditions and the reliability of ISD is lower than that of ISD ² in most studied conditions. The derived functions are applied to calculate the reliabilities of intraindividual variability indicators of negative affect measured over 56 days and perceptual-motor performance measured over five assessments. Implications for measurement-burst study designs are discussed.