  In quark-antiquark pair production at LEP, many features of the hadronic
final state can be predicted by QCD. Using data collected by the OPAL
experiment, we present the statistical distributions of fourteen "event shape
observables," which describe the inclusive kinematic properties of events
producing three or more jets. For six of these observables, we compare the
measured distributions with those calculated in perturbative QCD. By optimising
the agreement between theory and data, we measure the strong coupling
$\alpha_s$ at a range of energy scales. We also test the predictions of three
Monte Carlo event generators, for all fourteen observables. Over the years
since the LEP experiments began operating, many similar analyses have been
published, and have contributed to the world average measurements of
$\alpha_s$. However, several improvements have now been made, both in the
theoretical calculations and in the experimental analysis techniques. We
therefore present a complete reanalysis of the OPAL data, over the full range
of LEP collision energies. Particular attention is given to the estimation of
uncertainties, including the large contribution due to uncalculated
higher-order terms of the theory predictions. In collaboration with the LEP QCD
Working Group, we have combined the $\alpha_s$ measurements obtained from event
shape observables by all four experiments. Detailed investigations were
undertaken to ensure consistent implementation of the theoretical predictions
and uncertainty estimates, and to take account of correlations between
measurements.
