[R] residual df in lmer and simulation results

[Bill Shipley]

Hello.  Douglas Bates has explained in a previous posting to R why he does
not output residual degrees of freedom, F values and probabilities in the
mixed model (lmer) function:  because the usual degrees of freedom (obs -
fixed df -1) are not exact and are really only upper bounds.  I am
interpreting what he said but I am not a professional statistician, so I
might be getting this wrong...
Does anyone know of any more recent results, perhaps from simulations, that
quantify the degree of bias that using such upper bounds for the demoninator
degrees of freedom produces?  Is it possible to calculate a lower bounds for
such degrees of freedom?

Thanks for any help.

Bill Shipley
North American Editor, Annals of Botany
Editor, "Population and Community Biology" series, Springer Publishing
Département de biologie, Université de Sherbrooke,
Sherbrooke (Québec) J1K 2R1 CANADA
Bill.Shipley at USherbrooke.ca
http://pages.usherbrooke.ca/jshipley/recherche/