Likelihood Ratio
Definition
A statistical expression of the strength of evidence: how much more probable the observed findings are if the prosecution's hypothesis is true than if the defence's hypothesis is true. Communicating a likelihood ratio accurately to a lay tribunal is one of the core challenges of forensic testimony.
- Formula
- p(evidence given prosecution hypothesis) divided by p(evidence given defence hypothesis)
- Scale reading
- Greater than 1 favours prosecution hypothesis, less than 1 favours defence
- Statistical basis
- Bayesian inference
- Common uses
- DNA, fingerprint and voice comparison evidence
Common questions
How does a likelihood ratio differ from a probability of guilt?+
It measures how much the evidence shifts belief between two hypotheses, not the probability that a defendant is guilty. Conflating the two is the prosecutor's fallacy, a common courtroom error.
Why is a verbal scale sometimes used alongside the number?+
Numeric ratios like 10,000 to 1 are hard for juries to interpret intuitively, so many labs pair the figure with a verbal equivalent such as strong support, though critics argue the verbal bands can obscure real uncertainty.
Related terms
- Cross-Examination
- Questioning of a witness by the opposing party. For an expert, cross-examination probes qualifications, methodology, the basis of opinions, limitations, inconsistencies with...
- Daubert Gatekeeping
- The judicial function under Daubert v. Merrell Dow Pharmaceuticals (1993) and Federal Rules of Evidence Rule 702, requiring the trial judge to...
- Examination-in-Chief
- The questioning of a witness by the party who called them. For an expert, this is typically limited because the substance is...
- Expert's Duty to the Court
- The overriding obligation, recognised in common law and many civil law systems, that the expert's evidence must be honest, independent, and complete,...
- Pre-Trial Conference
- A meeting between the forensic expert and instructing counsel held before the hearing to agree on scope, clarify limitations, identify likely challenges,...