  It is emphasized that the bunching parameter $\beta=P_B/P_D$ , i.e. the ratio
between the probability to measure two bosons and two distinguishable particles
at the same state, is a constant of motion and depends only on the overlap
between the initial wavefunctions. This ratio is equal to $\beta=2/(1+I^2)$ ,
where $I$ is the overlap integral between the initial wavefunctions. That is,
only when the initial wavefunctions are orthogonal this ratio is equal to 2,
however, this bunching ratio can be reduced to 1, when the two wavefunctions
are identical. This simple equation explains the experimental evidences of a
beam splitter. A straightforward conclusion is that by measuring the local
bunching parameter $\beta$ (at any point in space and time) it is possible to
evaluate a global parameter$ I$ (the overlap between the initial
wavefunctions). The bunching parameter is then generalized to arbitrary number
of particles, and in an analogy to the two-particles scenario, the well-known
bosonic enhancement appears only when all states are orthogonal.
