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The National Study of Health and Growth (NSHG) is a surveillance system which monitors the growth of primary school children in England and Scotland, set up in 1972 following changes in the school milk and welfare system. However, the study contained few children from inner city areas or ethnic minorities. In 1982 its design was changed from one in which the same areas were surveyed every year, to two separate systems with areas surveyed every two years, one set of areas corresponding to those in the original study, and the other set consisting of inner city and ethnic minority areas. The precision of the estimates of trends in height and weight for each system was smaller than that of the original system, but by less than 50%, so that an overall gain in information was achieved. Studies of mixed longitudinal design are shown also to be generally, but not always, less efficient for estimating trends than independent cross-sectional surveys obtaining the same number of measurements.