  The measurement of azimuthal distributions in nucleus-nucleus collisions
relies upon the assumption that azimuthal correlations between particles result
solely from their correlation with the reaction plane (i.e. flow). We show that
at SPS energies, the ansatz is no longer valid, and two-particle correlations
due to momentum conservation, final state interactions or resonance decays
become of the same order as those arising from flow. This leads us to introduce
new methods to analyse collective flow, based on a cumulant expansion, which
enable to extract smaller values than those accessible to the standard
analysis.
