Classical statistics is often called frequentist, owing to the centrality of frequencies of events in classical procedures and the prominence of the frequentist interpretation of probability developed by von Mises. In this interpretation, chances are frequencies, or proportions in a class of similar events or items. They are best thought of as analogous to other physical quantities, like mass and energy. It deserves emphasis that frequencies are thus conceptually prior to chances . In propensity theory the probability of an individual event or item is viewed as a tendency in nature, so that the frequencies, or the proportions in a class of similar events or items, manifest as a consequence of the law of large numbers. In the frequentist theory, by contrast, the proportions lay down, indeed define what the chances are. This leads to a central problem for frequentist probability, the so-called reference class problem: it is not clear what class to associate with an individual event or item (cf.  Reichenbach 1949, Hajek 2007). One may argue that the class needs to be as narrow as it can be, but in the extreme case of a singleton class of events, the chances of course trivialize to zero or one. Since classical statistics employs non-trivial probabilities that attach to the single case in its procedures, a fully frequentists understanding of statistics is arguably in need of a response to the reference class problem. 