 A major difficulty with both of the mathematical conceptions of probative value that we have just examined is that for most evidence, obtaining the figures necessary for computing the likelihood ratio is problematic (Allen 1991: 380). Exceptionally, quantitative base rates data exist, as in our blood type example. Where objective data is unavailable, the fact-finder has to draw on background experience and knowledge to come up with subjective values. In our blood type example, a critical factor in computing the likelihood ratio was the percentage of the “suspect population” who had the same blood type as the accused. “Reference class” is the general statistical term for the role that the suspect population plays in this analysis. How should the reference class of “suspect population” be defined? Should we look at the population of the country as a whole or of the town or the street where the alleged murder occurred? What if it occurred at an international airport where most the people around are foreign visitors? Or what if it is shown that both the accused and the victim were at the time of the alleged murder inmates of the same prison? Should we then take the prison population as the reference class? The distribution of blood types may differ according to which reference class is selected. Sceptics of mathematical modelling of probative value emphasize that data from different reference classes will have different explanatory power and the choice of the reference class is open to—and should be subjected to—contextual argument and requires the exercise of judgment; there is no a priori way of determining the correct reference class. (On the reference class problem in legal factfinding, see, in addition to references cited in the rest of this section, Colyvan, Regan, and Ferson 2001; Tillers 2005; Allen and Roberts 2007.)