Interpreting PROC Mixed Influence Diagnostics

[Redleg]

I'm a business user focused on interpretation of results (so I can muck around with somebody else's code and have enough of a statistical background to be dangerous).  I am looking for help in interpretting the outputs of PROC MIXED Influence Diagnostics.  

1.  My approach initially is to look at Cook's D and use a cutoff of 4/n, DFFITS with a cutoff of 2SQRT(p/n)  and examine if RMSE changes much for levels with or without this level.  is that a valid approach?

2.  I read breifly somewhere that I could use PRESS to examine overfitting - does anybody have any direction they coud provide on how I should thinkg aobut this?

3.  How should I interpret the other measures provided?

4.  I have a model that provides no diagnostics, providing notes only of Loss of Rank or Change in Singularity.  Need help in interpetting this (but I fear this means that my model is fragile and cannot live without all the levels that I'm using.  

5.  Anything that I'm missing / not thinking of?

Help greatly appreciated

[lvm]

In addition to the examples in the MIXED chapter of the User's Guide (available on-line), I recommend that you read the following:

http://www2.sas.com/proceedings/sugi29/189-29.pdf