  We identify the correlation in a state of two identical particles as the
residual information beyond what is already contained in the 1-particle reduced
density matrix, and propose a correlation measure based on the maximum entropy
principle. We obtain the analytical results of the correlation measure, which
make it computable for arbitrary two-particle states. We also show that the
degrees of correlation in the same two-particle states with different particle
types will decrease in the following order: bosons, fermions, and
distinguishable particles.
