First, it is descriptively insufficient. The law requires the court to apply a fixed standard of proof for all cases falling within the relevant category. Theoretically, the same high standard applies to all criminal cases and the same lower standard applies to all civil cases. However, it is not clear whether investigators adhere strictly to a fixed standard of proof (cf. Kaplow 2012: 805-809). There is a second way of thinking about probative value. Under the second view, but not the first, the probative value of evidence is assessed in relation to the context. The probative value of E may be low in one state of the other line of evidence and significant in another piece of evidence (Friedman, 1986; Friedman and Park, 2003; see Davis and Follette, 2002, 2003). If the other evidence shows that a woman died when she fell down an escalator in a mall while shopping, it is unlikely that her husband`s story as domestic violence has any probative value to prove that he was responsible for her death. But if the other evidence shows that the wife died from injuries in the matrimonial home and the question is whether the injuries were sustained by an accidental fall from the stairs or inflicted by the husband, the same evidence of domestic violence will now have significant probative value. The reference class problem is not limited to the probabilistic assessment of the weight of evidence of individual evidence. This is a general difficulty with a mathematical approach to legal proof. The same problem arises in particular in a probabilistic interpretation of the standard of proof, where the court must determine, on the basis of all the evidence presented in this case, whether the test is met.
This topic is discussed in section 3.2 below, but it is useful at this point to illustrate how the reference class problem can also occur in this context. The plaintiff is suing Blue Bus Company for compensation for injuries sustained in an accident. The plaintiff testified, and the court believed, based on his testimony, that he was run over by a bus being driven recklessly. Unfortunately, it was dark at the time and he can`t tell if the bus belonged to the Blue Bus Company. Suppose further that there is also evidence that the Blue Bus Company owns 75% of the buses in the city where the accident occurred, and the remaining 25% belongs to the Red Bus Company. No further evidence is presented. The use of the data as a basis for concluding that there is a 0.75 probability that the bus involved in the accident belonged to the Blue Bus Company appears to favour the reference class of “city buses” over other possible reference categories such as “buses on the road where the accident occurred” or “buses in service at the time in question” (Allen and Pardo, 2007a: 109). Different reference classes can give very different probability ratios. How the reference class is chosen is crucial, and it is ultimately a matter of reasoning and judgment. Any choice of reference class (with the exception of the class that shares each characteristic of the particular incident, which is in fact the single incident itself) is in principle questionable. In countries that follow the civil justice system, evidence is generally examined as a branch of procedural law.
The second definition is found in the United States Federal Rule of Evidence 401, which (as revised) states that evidence is relevant if it “tends to make a fact more or less likely than it would be without the evidence” (emphasis added). The word “likely” in these and other standard definitions is sometimes interpreted as carrying the mathematical meaning of probability. [8] In an editorial, Lempert cited this example to show how relevance affects the probability ratio. The prosecution adduces evidence that the author`s blood found at the scene is type A. The accused has the same blood type. Suppose fifty percent of the suspect population has type A blood. If the defendant is indeed guilty, the probability that the blood found at the scene is type A is 1.0. But if he is indeed innocent, the probability of finding type A blood at the crime scene is 0.5 – that is, it corresponds to the background probability of type A blood of the suspect population. The probability ratio is the ratio of the first probability to the second – 1.0:0.5 or more simply 2:1. Evidence is considered relevant as long as the probability ratio is not 1:1 (Lempert 1977).
If the ratio is 1:1, it means that the probability of obtaining evidence is the same as if the accused is guilty or innocent. If an expert presents a scientific fact as substantial evidence or as the basis for his or her opinion, the court must establish the reliability of the scientific fact, taking into account such factors as the validity of the underlying scientific principle, the validity of the technique, the application of that principle, due process of law, the condition of the instruments used in the proceedings and their qualifications. who perform the test and interpret the results. Problems often arise with scientific tools and techniques such as lie detectors, DNA testing, and hypnosis.