 A related issue is how the evaluation of individual well-being must be made, or, equivalently, how interpersonal comparisons must be performed. There is now a growing interest in exploring concrete ways of measuring individual well-being, as illustrated by happiness studies (section 7.5 below) and the handbook published by Adler and Fleurbaey (2016). Welfare economics traditionally relied on “utility”, and the extended informational basis of social choice is mostly formulated with utilities (although the use of extended preference orderings is often shown to be formally equivalent: for instance, saying that Jones is better-off than Smith is equivalent to saying that it is better to be Jones than to be Smith, in some social state). But utility functions may be given a variety of substantial interpretations, so that the same formalism may be used to discuss interpersonal comparisons of resources, opportunities, capabilities and the like. In other words, one may separate two issues: 1) whether one needs more information than individual preference orderings in order to perform interpersonal comparisons; 2) what kind of additional information is ethically relevant (subjective utility or objective notions of opportunities, etc.). The latter issue is directly related to philosophical discussions about how well-being should be conceived and to the “equality of what” debate. This debate, which originates with Sen (1980), focuses on seeking the appropriate metric of advantage that should be used by theories of justice that are egalitarian or that give priority to the worst-off.