The Character of Z-pole Data Constraints on Standard Model Parameters
  Despite the impressive precision of the Z-pole measurements made at LEP and SLC, the allowed region for the principle Standard Model parameters responsible for radiative corrections (the mass of the Higgs, the mass of the top and alpha(Mz)) is still large enough to encompass significant non-linearities. The nature of the experimental constraints therefore depends in an interesting way on the "accidental" relationships among the various measurements. In particular, the fact that the Z-pole measurements favor values of the Higgs mass excluded by direct searches leads us to examine the effects of external Higgsstrahlung, a process ignored by the usual precision electroweak calculations. 
T. Kawamoto and R. G. Kellogg