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 one to measure the moments of the multiplicity distributions of protons (p), kaons (K), pions (π), and electrons (e) in case of incomplete particle identification. Using these moments the excitation function of the fluctuation measure ν[subscript dyn] [A,B] was measured, with A and B denoting different particle types. The obtained energy dependence of ν[subscript dyn] agrees with previously published NA49 results on the related measure σdyn. However, ν[subscript dyn] [K,p] and ν[subscript dyn] [K,π] were found to depend on the phase-space coverage. This feature most likely explains the reported differences between measurements of NA49 and those of STAR in central Au + Au collisions.United States. Dept. of Energy (Grant DE-FG03-97ER41020/A000)