  We explore the relation between proton and nucleon number fluctuations in the
final state in relativistic heavy ion collisions. It is shown that the
correlations between the isospins of nucleons in the final state are almost
negligible over a wide range of collision energy. This leads to a factorization
of the distribution function of the proton, neutron, and their antiparticles in
the final state with binomial distribution functions. Using the factorization,
we derive formulas to determine nucleon number cumulants, which are not direct
experimental observables, from proton number fluctuations, which are
experimentally observable in event-by-event analyses. With a simple treatment
for strange baryons, the nucleon number cumulants are further promoted to the
baryon number ones. Experimental determination of the baryon number cumulants
makes it possible to compare various theoretical studies on them directly with
experiments. Effects of nonzero isospin density on this formula are addressed
quantitatively. It is shown that the effects are well suppressed over a wide
energy range.
