Figure 1.

Illustration of four common variance–covariance structures. When six measurement time points are considered, the top panel illustrates the within experimental unit variances assumed at each time point under the four variance–covariance structure. The bottom panel illustrates corresponding correlation matrices among the measurement time points. Color gradients are used to show the magnitude of the variances and the strength of the correlations. The darker the color grade is, the larger the variance is or the stronger the correlation is.