SITAR growth curve analysis - a statistical method to assess longitudinal growth and development data

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The pattern of growth in individual children, as represented by their growth curve of successive measurements (e.g. weight or height) plotted against age, may be informative about their circumstances at earlier ages, or conversely it may predict their risk of disease later in life, for example heart disease, stroke or type 2 diabetes. To investigate this it is useful to summarise the shape of each individual's growth curve in a simple way, so that its shape can be related to their earlier or later experiences. The SITAR method of growth curve analysis is a recently developed approach that does just this - it summarises each individual growth curve as just three numbers, reflecting respectively their size (i.e. whether they are tall or short, or heavy or light, as compared to others), their growth tempo (i.e. whether they are relatively mature or delayed in their timing of growth, e.g. when girls reach menarche), and velocity (how fast they are growing compared to others, e.g. in puberty). So for example an individual might be relatively tall, with early puberty, and growing relatively fast. In addition SITAR estimates a smooth average growth curve against which each individual's growth pattern can be compared. The surprising thing is that this model explains nearly all of the variability in growth between individuals, so that their size, tempo and velocity can be viewed as a complete summary of their growth pattern. Put another way, when adjusted for size, tempo and velocity all their growth curves look the same (the SITAR acronym mean Super-Imposition by Translation And Rotation). It is then simple to relate the values of size, tempo and velocity in individuals to their earlier or later experiences, and to look for links with their growth pattern.