  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.
