 Though it might seem that this goes without saying, the entire analysis is predicated on the assumption that, of the potential outcomes of a given experiment, one and only one occurs, and hence that it makes sense to speak of the outcome of an experiment. The reason that this assumption is worth mentioning is that there is a family of approaches to the interpretation of quantum mechanics, namely, Everettian, or “many-worlds” approaches, and some variants of the relational approach, hold that all outcomes occur in what are effectively distinct worlds. See entries on  many-worlds interpretation of quantum mechanics,  Everett’s relative-state formulation of quantum mechanics, and  relational quantum mechanics 