SAS PROC MIXED: Repeated statement

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The repeated statement specifies the error structure for each subject.

  repeated artime / type=ar(1) sub=id;

This example tells SAS to model serial correlation (in the form of a 1st order autoregressive process) within each subject, where the variable id is unique per subject. This is only appropriate when the samples are equally spaced. Also, missing values should be included with a missing value code (.) if ar(1) is used. The variable specified after the word repeated must be a categorical variable (i.e. specified in the class statement). If the samples are not equally spaced, a model analagous to ar(1) is the spatial power model. For example

  repeated / type=SP(POW)(time) sub=id;

models the correlation as falling off exponentially with the time between samples.

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