Roemer’s mechanism requires that the population is partitioned into types. Each type is characterized by a vector of socioeconomic and personal characteristics such as innate intelligence, race and sex. The set of characteristics reflects society’s view of which factors affect a person’s choices and over which one has no control. Given this partitioning into types, his mechanism can be seen as consisting of two parts: an objective function and a procedure to define comparable degrees of responsibility. People are said to have exercised a comparable degree of responsibility if they are at the same percentile of their type’s distribution of the outcome under consideration, say welfare. The objective is to maximize a weighted average of minimum welfares across types of persons of the same degree of responsibility. The weight attached to the welfare of those at a given degree of responsibility is their population frequency. It is the combination of these two components which allows the mechanism to be applied to actual situations. Given that one can observe people’s outcomes and types, no other information (such as counter-factual choices) is needed to implement the mechanism. From a practical point of view this is an enormous advantage.