In this chapter we introduce the basic concepts necessary to analyse individual series of growth measurements.
We also indicate where, in this volume, related topics are discussed.
Growth in auxology is defined as the increase in size of a body dimension and stands somewhat in opposition to development, which, usually, implies a qualitative change. The distinction is, however, not always clear cut, because the differential growth of different body dimensions can lead to qualitative changes, e.g. in proportion and shape, which well deserve the name of development.
Both in the study of growth, as well as of development, the notion of change is central. This has implications for the design of a growth study: only a minimal amount of information about change can be gathered from cross-sectional studies, namely merely the average change. Short mixed cross-sectional/longitudinal studies provide information about short-term growth trends, often relevant for clinical purposes; however a picture of the growth process as a whole and a correct view of the relationship between different growth phases can only be obtained from time-consuming longitudinal studies, where subjects are followed and measured over many years, most desirably from birth, or even earlier, to, or well into, adulthood. Such studies are very delicate in their management, if selection bias is to be avoided, and measurement occasions must be designed carefully, essentially with smaller intervals where the expected change is more rapid.