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), th at, since 1995, has shown a deviation from the Standard Model prediction of between 2.4 and 3.1 standard deviations. Issues addressed include: The eff ect of particular measurements, values found by individual experiments, LEP /SLD comparison, and the treatment of systematic errors. The effect, curren tly 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 esse ntially the full LEP and SLD Z-decay data sets are now analyzed the meaning of the deviation, (new physics or marginal statistical fluctuation) is unl ikely to be given by the present generation of colliders.