2 GRAPHICAL TECHNIQUES USEFUL IN DETECTING CORRELATION STRUCTURE IN REPEATED-MEASURES DATA

Analysis of repeated measures data using a mixed model includes specif ying a form for the covariance matrix of the within-subject observatio ns. This reduction in the number of estimated parameters from the unsp ecified structure may improve the efficiency of inferences made. An im plementation of this technique has been incorporated in the MIXED proc edure of the SAS(R) statistical package, and includes a wide range of options for the structure of the covariance matrix. It is demonstrated that, draftman's display plots and/or plots in a coordinate system wi th parallel axes can aid in visualizing the dispersion structure.