GROWTH OF CHILDREN: CROSS-SECTIONAL STUDIES (Vinod Anand) In a cross-sectional study all the children are measured once. Data are obtained, for example, from a school population of children varying in age from 6 to 28 years. The end result of such a study gives information on a group of 6-year-olds, a group of 7-year-olds, and so on for each age group. When all the data on the 6-year-olds are averaged, a figure of what the average 6-year- old has accomplished in 6 years is obtained. The meaning of this figure depends upon how homogeneous the group of 6-year-olds is. Size at age 6 varies with genetic background, with socioeconomic level, with physical and emotional health, and probably with other factors as well. If the population measured contains children who fall in many of these different categories, the average figure is not of much value as a standard against which to compare the accomplishments of any given child in 6 years. Average figures from a cross- sectional study are useful in comparing groups. The average size of 6-year-olds in a New York City public school can be compared with the average size of the 6-year-olds in a Japanese school or a French school or a Mississippi school or in an English school in 1850. In a cross-sectional study the figures represent a static phenomenon. They are not a continuum, because no child moved from one group to the next. In many cross-sectional growth studies the data are used as though they were a continuum. The average figures of distance attained at the various ages are put together, and the resulting curve is called a growth curve (though nothing grewall was static). In using cross-sectional data in this way the assumption is made that all children of age 6, when the study was made, will be of the same ii size 1 year hence as those age 7 at the time of measurement. This assumption is not valid; children do not progress at the same rate at different ages. Differences in the rate of progression exist at all ages, but during the spurt of growth at puberty the differences are very marked indeed. Each child grows rapidly for 1 or 2 years during his period of sexual maturation. However, there is about an 8-year span during which individual children pass through this growth spurt. Girls mature earlier than boys, but even within the same sex 4 there is a range of several years during which maturation may take place. A girl who has not started to mature may grow 1 in. in height between 11 and 12 years of age; another girl in early puberty at this age may grow 5 in. in this same chronological time. The difference obtained in a cross-sectional study of height between the average 11-year-old girl and the average 12-year-old girl gives a figure which distorts the amount of gain of the early- and late maturing girls. Valuable information can be obtained from cross-sectional studies, but such studies are of relatively little value in studying the velocity of growththe increment per unit of time.