Some Notes on Refereeing |
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| Authors: | Leon J Gleser |
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| Institution: | Purdue University , West Lafayette , IN , 47907 , USA |
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| Abstract: | Analysis of longitudinal data using a general linear mixed model requires the specification of a form for the covariance matrix of within-subject observations. Graphical diagnostics, such as the scatterplot matrix, can be of substantial help in making this specification. I introduce another graphical diagnostic, the Partial-Regression-on-Intervenors Scatterplot Matrix (PRISM), which complements the ordinary scatterplot matrix and which is more useful for identifying certain kinds of correlation structures. PRISMs corresponding to several commonly used correlation structures are displayed. The PRISM's usefulness in model specification is illustrated with an example of longitudinal data from a 100-kilometer road race. |
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| Keywords: | Antedependence Autoregressive model Covariance matrix Draftsman's display Graphical diagnostic Mixed model PROC MIXED Random coefficients Scatterplot matrix |
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