Error estimate for the valence approximation and for a systematic expansion of full QCD

We construct a systematic expansion for the lattice formulation of full QCD. The leading term is the valence (quenched) approximation. An error estimate for any truncation of this expansion is then obtained from a second expansion of full QCD, indexed by the number of virtual fermion loops included in each term. {copyright} {ital 1997} {ital The American Physical Society}