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Even Wikipedia, where articles about statistical terms are usually taken over by formal mathematicians and their terminology, passes the buck with its definition referring to a “measure of likelihood.” Some theorists say that probabilities must refer to the future, though an ordinary person would probably think it reasonable to consider probabilities for some prior events (e.g. if you run down to a subway platform and hop on a train without looking at any signs, you might think it worthwhile to calculate the probability that you boarded the correct train).  In fact, most needs are served by an operational definition something like this: the probability of an event is the proportion of times it will occur if you could repeat (even if only in the imagination) the circumstances surrounding the event over and over innumerable times.