INTRODUCTION

The first longitudinal growth study dates back to 1759 when Count de Montbeillard measured the body length of his son from birth to 18 years (Scammon, 1927; Tanner, 1962). Actually, when studying growth, there are two basically different approaches: longitudinal and crosssectional studies. In longitudinal growth studies, we measure the same children over several years at regular intervals (as was done by de Montbeillard) in order to be able to establish individual growth patterns. In cross-sectional growth studies, we measure children of different ages only once. A plot of the average height obtained at each age (or age group) depicts the average growth pattern in the sample. One should realize that the shape of the curve seen in an average growth pattern is different from the shape of individual growth curves (Hauspie, 1989). The information provided by the longitudinal and cross-sectional approaches is quite different. Both methods have their advantages and limitations. Whether the data concerns individual or average growth patterns, we are dealing with a series of measures of size (height or average height, for example) at particular ages, either precise chronological ages (in case of longitudinal studies) or mid-points of age classes (in case of cross-sectional studies). However, the researcher is quite often interested in determining the underlying continuous process of growth, from which he wants to derive certain characteristics, such as the age at maximum velocity at adolescence, for example.