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 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 σ level, is found to vary in the range from 1.7 σ to 2.9 σ 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.