  A model independent analysis has been performed on the LEP and SLD data on Z
decays. Using only very weak theoretical assumptions, the effective vector and
axial-vector couplings of leptons, c quarks and b quarks have been extracted.
Although the lepton and c quark couplings agree well with Standard Model
predictions, those of the b quark show deviations of more than three standard
deviations. The effect is mainly in the right-handed b quark coupling, the
left-handed coupling being consistent (at the 2$\sigma$ level) with the
Standard Model prediction. The probability that the observed deviations of all
the measured effective couplings are statistical fluctuations from lepton
universality and the Standard Model is estimated to be 0.9%. The estimated
probability that the deviations in the leptonic and b quark couplings alone are
a fluctuation is 0.18%. A thorough discussion is made of the internal
consistency of the different measurements contributing to the average values
$A_l$ and $A_b$ used to extract the b quark couplings, as well as possible
sources of systematic error that may not, hitherto, have been taken into
account. Excluding \tpol~ measurements, which show internal inconsistencies,
from the averages increases the deviations of the extracted b quark couplings
from the Standard Model predictions to the four standard deviation level.
