The S matrix ansatz is a rigorously model independent approach to describe the cross-sections and asymmetries in e+ e- annihilation. Using the cross-sections and asymmetries measured with the L3 detector during the 1990 and 1991 running period, we determine the mass and the width of the Z boson, the contributions of the Z exchange and of the gammaZ interference. Including the polarization of the tau lepton in the analysis, the leptonic helicity amplitudes of the scattering process are determined assuming lepton universality. The results are compared with other model independent ansatzes as realized in ZFITTER. A systematic bias of the Z mass due to the gammaZ interference term is detected, which leads to an underestimation of the error on m(Z) for model independent determinations.