The nature and meaning of intraindividual variability in development in the early life span

It has long been acknowledged that individual development is almost never smooth, but that it shows many irregularities. Though intraindividual variability seems to be a rather universal finding, its importance has not always been recognized. Traditionally, the most common theoretical explanations are (a) that variability is a result of context changes or (b) that variability is caused by measurement error.