  The Single-instanton approximation (SIA) is often used to evaluate
analytically instanton contribution to euclidean correlation functions in QCD,
at small distances. We discuss how this approximation can be consistently
derived from the theory of the instanton ensemble and give precise definitions
to a number of different "effective masses", generalizing the parameter $m*$,
which was introduced long ago to account for the collective contribution of the
whole ensemble.
  We test numerically the range of applicability of the SIA for different
quantities. Furthermore, we determine all the effective masses (for random and
interacting instanton liquid models) as well as from phenomenology, and discuss
to what extent those are universal.
