A Comparison of Efficiencies of Longitudinal, Mixed Longitudinal, and Cross-Sectional Designs

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Abstract
The choice among a longitudinal, mixed longitudinal, or cross-sectional design is often called for in educational and psychological research. The problem of choosing the most efficient design to estimate polynomial parameters for time-structured data is considered, and the comparison of the efficiencies shows that the assumed degree of the polynomial is crucial for the selection of the most efficient design. When the degree is much smaller than the number of time points and the correlations between adjacent time points are not too large, cross-sectional and mixed longitudinal designs are more efficient than a longitudinal design.