  A novel approach, the identity method, was used for particle identification
and the study of fluctuations of particle yield ratios in Pb+Pb collisions at
the CERN Super Proton Synchrotron (SPS). This procedure allows to unfold the
moments of the unknown multiplicity distributions of protons (p), kaons (K),
pions ($\pi$) and electrons (e). Using these moments the excitation function of
the fluctuation measure $\nu_{\text{\text{dyn}}}$[A,B] was measured, with A and
B denoting different particle types. The obtained energy dependence of
$\nu_{\text{dyn}}$ agrees with previously published NA49 results on the related
measure $\sigma_{\text{dyn}}$. Moreover, $\nu_{\text{dyn}}$ was found to depend
on the phase space coverage for [K,p] and [K,$\pi$] pairs. This feature most
likely explains the reported differences between measurements of NA49 and those
of STAR in central Au+Au collisions.
