  Particle number fluctuations and correlations in nucleus-nucleus collisions
at SPS and RHIC energies are studied within the statistical hadron-resonance
gas model in different statistical ensembles and in the Hadron-String-Dynamics
(HSD) transport approach. Event-by-event fluctuations of the proton to pion and
kaon to proton number ratios are calculated in the HSD model for the samples of
most central collision events and compared with the available experimental
data. The role of the experimental acceptance and centrality selection is
discussed.
