 The reference class problem is not confined to the probabilistic assessment of the probative value of individual items of evidence. It is a general difficulty with a mathematical approach to legal proof. In particular, the same problem arises on a probabilistic interpretation of the standard of proof when the court has to determine whether the standard is met based on all the evidence adduced in the case. This topic is explored in  section 3.2  below but it is convenient at this juncture to illustrate how the reference class problem can also arise in this connection. Let it be that the plaintiff sues Blue Bus Company to recover compensation for injuries sustained in an accident. The plaintiff testifies, and the court believes on the basis of his testimony, that he was run down by a recklessly driven bus. Unfortunately, it was dark at the time and he cannot tell whether the bus belonged to Blue Bus Company. Assume further that there is also evidence which establishes that Blue Bus Company owns 75% of the buses in the town where the accident occurred and the remaining 25% is owned by Red Bus Company. No other evidence is presented. To use the data as the basis for inferring that there is 0.75 probability that the bus involved in the accident was owned by Blue Bus Company would seem to privilege the reference class of “buses operating in the town” over other possible reference classes such as “buses plying the street where the accident occurred” or “buses operating at the time in question” (Allen and Pardo 2007a: 109). Different reference classes may produce very different likelihood ratios. It is crucial how the reference class is chosen and this is ultimately a matter of argument and judgment. Any choice of reference class (other than the class that shares every feature of the particular incident, which is, in effect, the unique incident itself) is in principle contestable.