covariance structures

The Unstructured Covariance Matrix: When It Does and Doesn’t Work

June 22nd, 2012 by

If you’ve ever done any sort of repeated measures analysis or mixed models, you’ve probably heard of the unstructured covariance matrix. They can be extremely useful, but they can also blow up a model if not used appropriately. In this article I will investigate some situations when they work well and some when they don’t work at all.

The Unstructured Covariance Matrix

The easiest to understand, but most complex to estimate, type of covariance matrix is called an unstructured matrix. Unstructured means you’re not imposing any constraints on the values.  For example, if we had a good theoretical justification that all variances were equal, we could impose that constraint and have to only estimate one variance value for every variance in the table.

But in an unstructured covariance matrix there are no constraints. Each (more…)