Choosing a Correlation Structure

The Unstructured correlation is the most general structure, but perhaps it's not the best. You can compare it to an Independent structure by comparing the QIC values obtained by each model.

  1. Recall the Generalized Estimating Equations dialog.
    Figure 1. Generalized Estimating Equations Repeated tab
    Generalized Estimating Equations Repeated tab
  2. On the Repeated tab, select Independent from the Structure drop-down list in the Working Correlation Matrix group.
  3. Click OK.
Figure 2. Goodness of fit for Independent model
Goodness of fit for Independent model

The QIC for the model with the Independent correlation matrix is 1829.280, which is larger than the QIC of 1829.275 for the model with the Unstructured matrix. The Unstructured correlation structure appears to provide a better model, though the tiny difference in the values of the fit statistics does not provide much guidance one way or the other.

Figure 3. Parameter estimates for Independent model
Parameter estimates for Independent model

In GEE, the parameter estimates should be consistent even if the working correlation structure is improperly specified. For this data set, the regression parameter estimates are similar for both working correlation structures.

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