 Allen and Pardo (2007) argue that the reference class problem poses a challenge for legal probabilism and more specifically for probabilistic measures of evidentiary strength such as likelihood ratios (see earlier in  Section 2).  The problem is that the same piece of evidence may be assigned different likelihood ratios depending on the reference class chosen. For example, the denominator in the likelihood ratio associated with a DNA match is the probability of a match given that a random person, unrelated to the crime, is the source. This probability depends on the frequency of the profile in a select population. But which reference population should one choose? Since nothing in the natural world picks out one reference class over another—the argument goes—the likelihood ratio would be an arbitrary measure of evidentiary strength.