  We consider the change of entanglement of formation $\Delta E$ produced by a
unitary transformation acting on a general (pure or mixed) state $\rho$
describing a system of two qubits. We study numerically the probabilities of
obtaining different values of $\Delta E$, assuming that the initial state is
randomly distributed in the space of all states according to the product
measure introduced by Zyczkowski {\it et al.} [Phys. Rev. A {\bf 58} (1998)
883].
