Measurements of distributions and mean values of the event-shape variables thrust and heavy jet mass in the center-of-mass energy range from 14 GeV up to 172 GeV are analyzed for possible non-perturbative corrections following a power law of the form 1/Q. These corrections are characterized by a single parameter, which can be fitted to the data together with the strong coupling constant. Results on alpha_s are compared to those obtained with hadronization corrections from a Monte Carlo generator
