  The experimental data from quasielastic electron scattering from $^{12}$C are
reanalyzed in terms of a new scaling variable suggested by the interacting
relativistic Fermi gas with scalar and vector interactions, which is known to
generate a relativistic effective mass for the interacting nucleons. By
choosing a mean value of this relativistic effective mass $m_N^* =0.8 m_N$, we
observe that most of the data fall inside a region around the inverse
parabola-shaped universal scaling function of the relativistic Fermi gas. This
suggests a method to select the subset of data that highlight the quasielastic
region, about two thirds of the total 2,500 data. Regardless of the momentum
and energy transfer, this method automatically excludes the data that are not
dominated by the quasielastic process. The resulting band of data reflects
deviations from the perfect universality, and can be used to characterize
experimentally the quasielastic peak, despite the manifest scaling violation.
Moreover we show that the spread of the data around the scaling function can be
interpreted as genuine fluctuations of the effective mass $M^* \equiv m^*_N/m_N
\sim 0.8 \pm 0.1$. Applying the same procedure we transport the scaling
quasielastic band into a theoretical prediction band for neutrino scattering
cross section that is compatible with the recent measurements and slightly more
accurate.
