A Study of the LEP and SLD Measurements of $A_{b}$

A systematic study is made of the data dependence of the parameter $A_b$, that, since 1996, has shown a deviation from the Standard Model prediction of between 3.3 and 2.5 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 2.5$\sigma$ 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 LEP1 and SLD 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.