  We elaborate on a recently suggested effective Lagrangian for charged-current
and neutral-current electroweak interactions which in comparison with the
standard electroweak theory contains three free parameters $\Delta x, \Delta
y$, $\varepsilon$ which quantify different sources for violations of $SU(2)$
symmetry. Within the standard $SU(2)_I \times U(1)_Y$ electroweak theory, we
present both exact and very much refined approximate analytical one-loop
expressions for these parameters in terms of the canonical input, $G_\mu, \MZ$,
$\alpz$, the top-quark mass, $\Mt$, and the Higgs-boson mass, $\MH$. We
reemphase the importance of discriminating between the {\it empirically
well-known purely fermionic} (vacuum polarization) contributions to $\Delta x,
\Delta y$, $\varepsilon$ and the {\it empirically unknown bosonic} ones with
respect to present and future electroweak precision tests. The parameters
$\Delta x$ and $\varepsilon$ are hardly affected by standard bosonic
corrections, while the full one-loop results for $\Delta y$ differ appreciably
from the ones obtained by taking into account fermion loops only. A detailed
comparison with the experimental data on $\MWpm/\MZ$, $\bar\sw^2$, $\Gamma_l$
shows that these data start to become accurate enough to be sensitive to
standard (bosonic) contributions to $\Delta y$ beyond fermion loops.
