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    A Study of the LEP and SLD Measurements of AbA_b

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    A systematic study is made of the data dependence of the parameter AbA_{\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 2.4σ\sigma level, is found to vary in the range from 1.7σ\sigma to 2.9σ\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.Comment: 15 pages 7 figures 7 table