Interaction plot between 2 continuous variables
Bert Gunter <gunter.berton <at> gene.com> writes:
This almost certainly has nothing to do with mixed effects models per se. x1 and x2 are probably ( if what you see is not due to a few unusual values) correlated, so the standard decomposition into main and interaction effects does not have the usual meaningful interpretation (e,g, that you would get from orthogonal regressors).
See also http://stats.stackexchange.com/questions/5450/ what-if-interaction-wipes-out-my-direct-effects-in-regression/ (URL broken to make gmane happy)