Аннотация:The use of four gauge invariant form factors allows a dense notation of electroweak one loop effects in muon pair production frome + e −-annihilation. These form factors may be interpreted as finite renormalizations of the muon decay constant and of the weak mixing angle. Such a formulation is well adopted for use in Monte Carlo programs. The general formulae are applied to a realistic line shape scan of theZ boson resonance. For a precise determination of theZ boson's mass and width it is sufficient to take data in the small energy region within . Finally, it is demonstrated that a simple model independent ansatz yields nearly equally good results.