Connecting the Quenched and Unquenched Worlds via the Large N_c World
  In the large N_c(number of colors) limit, quenched QCD and QCD are identical. This implies that, in the effective field theory framework, some of the low energy constants in (N_c=3) quenched QCD and QCD are the same up to higher-order corrections in the 1/N_c expansion. Thus the calculation of the nonleptonic kaon decays relevant for the Delta I=1/2 rule in the quenched approximation is expected to differ from the unquenched one by an O(1/N_c) correction. However, the calculation relevant to the CP-violation parameter epsilon'/epsilon would have a relatively big higher-order correction due to the large cancellation in the leading order. Some important weak matrix elements are poorly known that even constraints with 100% errors are interesting. In those cases, quenched calculations will be very useful. 
Jiunn-Wei Chen