Reference Class
Definition
The population against which a probability or frequency is calculated. Choosing the wrong reference class, for example using general population allele frequencies rather than frequencies within the relevant ethnic subgroup, produces misleading statistics.
- Defines
- The comparison population for a probability calculation
- Common error
- Using general population data instead of a relevant subgroup
- Field
- Forensic statistics, especially DNA and other frequency evidence
Common questions
Why does the choice of reference class change the reported statistic?+
Allele or feature frequencies vary between subpopulations, so the same evidentiary match can yield a very different random-match probability depending on which population's frequency data is used as the denominator. Choosing an inappropriate class can overstate or understate the strength of the evidence.
Who decides which reference class is appropriate in a given case?+
The examiner selects it based on the case circumstances and available population data, and it is a frequent point of cross-examination and expert disagreement in court because there is often no single objectively correct choice, only a more or less defensible one.
Related terms
- Defence Fallacy
- The converse error of inflating the importance of the RMP by arguing that, because many people in the population share the profile,...
- Likelihood Ratio (LR)
- The ratio of two conditional probabilities: the probability of the observed evidence given the prosecution's hypothesis (same source), divided by the probability...
- Product Rule (Probability)
- The rule that P(A and B) = P(A) x P(B) holds only when A and B are independent. Applying it to correlated...
- Prosecutor's Fallacy
- The error of treating the RMP (or its reciprocal) as the probability that the defendant is innocent, or as the probability that...
- Transposition of the Conditional
- The mathematical name for the error at the core of the prosecutor's fallacy. P(A | B) and P(B | A) are generally...