The paper addresses the issue of measuring web accessibility in such a way that differences in measurements reflect differences in the effectiveness experienced by disabled users. The paper presents the steps upon which a measuring methodology called MAMBO is based, the data that are needed to compute the indexes, in addition to its conceptual rationale. An experimentation of MAMBO is then described, based on analysis of 14 accessibility reports; results are shown and discussed, including the effects that different severity judgments may have on the metric, how to estimate confidence intervals on the values, how the metric can be used to estimate accessibility with respect to specific user groups.
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