IIV:

"for more than 80 years (Fiske & Rice 1955), [psychologists] have accumulated evidence that nonsystematic variability in behaviour (i.e. IIV) is not simply random ‘noise’, but that it varies as a result of age and prior experience, may significantly vary across individuals, and may be stable across time within individuals (Nesselroade 1991; Siegler 2007; Ram & Gerstorf 2009; Salthouse & Nesselroade 2010)."

Judy A. Stamps et al. - Unpredictable animals