Prior probability
Definition
The probability of guilt (or innocence) based on all evidence other than the specific forensic match under consideration. Bayes' theorem requires a prior to convert a likelihood ratio into a posterior probability. Courts, not experts, are responsible for assessing the prior in a criminal case.
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...
- 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...
- Random match probability (RMP)
- The probability that a randomly chosen unrelated person from the relevant population would match the evidence profile by chance. A very small...
- Relevant population
- The population of individuals who could plausibly have left the forensic sample, constrained by geography, time, and other case facts. Choosing the...
- 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...
Explained in these topics
- The Defence FallacyThe probability of a hypothesis before the forensic evidence is considered, derived from all other evidence in the case: witness accounts, CCTV, alibi, motive,...
- The Prosecutor's FallacyThe probability of guilt (or innocence) based on all evidence other than the specific forensic match under consideration. Bayes' theorem requires a prior to co...