  We derive the relation between cumulants of a conserved charge measured in a
subvolume of a thermal system and the corresponding grand-canonical
susceptibilities, taking into account exact global conservation of that charge.
The derivation is presented for an arbitrary equation of state, with the
assumption that the subvolume is sufficiently large to be close to the
thermodynamic limit. Our framework -- the subensemble acceptance method (SAM)
-- quantifies the effect of global conservation laws and is an important step
toward a direct comparison between cumulants of conserved charges measured in
central heavy ion collisions and theoretical calculations of grand-canonical
susceptibilities, such as lattice QCD. As an example, we apply our formalism to
net-baryon fluctuations at vanishing baryon chemical potentials as encountered
in collisions at the LHC and RHIC.
