  The precision of measurements of the strong coupling constant using
event-shape variables in e+e- annihilations is limited by theoretical
systematic uncertainties. The uncertainties are related to missing higher
orders in the perturbative predictions for the event-shape distributions. A new
method is presented for the assessment of theoretical uncertainties in alpha_s.
This method evaluates the systematic uncertainty of the parameter alpha_s from
the uncertainty of the prediction for the distributions from which it is
extracted. The perturbative uncertainties are calculated on a purely
theoretical basis, without accessing measured distributions. The method is
therefore especially suited for an unbiased combination of results from
different observables or experiments. It is universal and can be applied to
other processes like jet production in deep-inelastic ep scattering or in
hadron collisions.
