Mixed longitudinal studies: their efficiency for the estimation of trends over time

Mixed longitudinal studies may be employed for the estimation of trends over time because they give more comparable data than a series of cross-sectional studies, or because of multiple objectives. The estimation of trends from such studies is an example of their use for a cross-sectional rather than longitudinal objective, for which the appropriate analysis is different from that usually described for the analysis of longitudinal data. The efficiency of mixed longitudinal studies for measuring trends in measurements, such as height and weight relative to a comparable series of cross-sectional studies, is calculated for various designs, percentage follow-up and correlation between measurements on the same subjects. The relative efficiency was over 0.8 for most designs and sometimes greater than 1.0. It generally decreased as the number of surveys increased, but increased as the difference between age groups studied increased.