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 ... View more