  A systematic study is made of the data dependence of the parameter
$A_{\rm{b}}$, that, since 1995, has shown a deviation from the Standard Model
prediction of between 2.4 and 3.1 standard deviations. Issues addressed
include: the effect of particular measurements, values found by individual
experiments, LEP/SLD comparison, and the treatment of systematic errors. The
effect, currently at the 2.4$\sigma$ level, is found to vary in the range from
1.7$\sigma$ to 2.9$\sigma$ by excluding marginal or particularly sensitive
data. Since essentially the full LEP and SLD Z decay data sets are now analysed
the meaning of the deviation, (new physics, or marginal statistical
fluctuation) is unlikely to be given by the present generation of colliders.
