Prior Odds
Definition
The ratio of the probability of the prosecution hypothesis to the probability of the defence hypothesis before the forensic evidence in question is considered. Set by the fact-finder using all non-forensic case information. The forensic expert does not supply this quantity.
- Formula role
- O(Hp) in Bayes' odds form: O(Hp|E) = O(Hp) x LR
- Set by
- The fact-finder (judge or jury), not the forensic expert
- Based on
- Non-forensic case information: witness accounts, opportunity, motive
- Combines with
- The likelihood ratio to produce posterior odds
Common questions
Why is the forensic expert barred from stating prior odds?+
Prior odds depend on the whole case, including evidence the expert has not seen and is not qualified to weigh, such as alibi credibility or motive. An expert who assigns prior odds strays into the fact-finder's role and risks the ultimate-issue objection.
What happens if prior odds are never made explicit in court?+
The likelihood ratio can still be presented on its own, letting the fact-finder combine it informally with their own view of the other evidence. Explicit numerical priors are more common in written Bayesian network analyses than in oral testimony.
How do prior odds interact with a very strong likelihood ratio?+
Even a large likelihood ratio cannot overcome extremely low prior odds on its own, since posterior odds are the product of the two. This is why a rare-event match statistic alone is never proof of guilt without considering the rest of the case.
Related terms
- 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...
- Posterior Odds
- The ratio of the probability of the prosecution hypothesis to the probability of the defence hypothesis after the forensic evidence is taken...
- 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...
- Evaluative Reporting
- A framework for expressing forensic conclusions in terms of the probability of the evidence given the prosecution hypothesis versus the defence hypothesis...
- 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...
- Bayes' Theorem
- The mathematical rule that relates prior odds, the likelihood ratio, and posterior odds. In the odds form: O(Hp|E) = O(Hp) x LR....
- Defence Attorney's Fallacy
- The incorrect argument that a large global or national population means many unrelated individuals could share the profile, while ignoring the geographic...
- Defence Hypothesis (Hd)
- The proposition advanced by the defence, typically that the material came from an unknown unrelated individual or that the defendant was not...
- ENFSI DNA Working Group
- The European Network of Forensic Science Institutes DNA Working Group. Coordinates European forensic DNA standards, including the evaluative reporting guidelines adopted by...
- Hardy-Weinberg Equilibrium (HWE)
- The expected genotype frequency in a large, randomly mating population with no selection, mutation, or migration. Under HWE, the frequency of a...
- Product Rule
- The rule that, under independence, the joint probability of a multi-locus genotype equals the product of the individual locus genotype frequencies. Used...
- Prosecution Hypothesis (Hp)
- The proposition advanced by the prosecution, typically that the defendant is the source of the recovered material or was present at the...
Explained in these topics
- Bayes Theorem in Evidence Evaluation
- The Likelihood Ratio: Definition and LogicThe odds that the prosecution hypothesis is true before the forensic evidence is considered. In court, prior odds reflect all evidence other than the specific...
- Prior and Posterior Odds in CourtThe ratio of the probability of the prosecution proposition (Hp) to the probability of the defence proposition (Hd) before any forensic evidence is considered....
- Allele Frequencies, Random Match Probability and the Likelihood RatioThe odds of the prosecution hypothesis relative to the defence hypothesis before the DNA evidence is considered. Combined with the LR by Bayes' theorem to yiel...