Re: Missing values in MIXED

Posted by Bruce Weaver on Mar 17, 2013; 1:16pm
URL: http://spssx-discussion.165.s1.nabble.com/Missing-values-in-MIXED-tp5718714p5718753.html

Ryan, I think you meant to say more restrictive below, did you not?  I.e., unstructured imposes no restrictions on the covariance matrix, and therefore uses up more df.  When you use another structure that fits the data reasonably well, you impose restrictions that buy you some df.

Cheers,
Bruce

Ryan Black wrote

--- snip ---

> I NOW recommend MIXED with UNSTRUCTURED covariance matrix across the board.

That is a poor recommendation. The goal should be to find the optimal residual variance-covariance structure. You could reduce statistical power if you employ an unstructured matrix if there is a less restrictive structure that fits that data equally well (e.g., AR1. TOEP). There may be other aspects to your data as well (G-side random effects that should be incorporated). [emphasis added]