Defence Fallacy
Definition
The converse error of inflating the importance of the RMP by arguing that, because many people in the population share the profile, the evidence has no weight. This fails because the relevant population is not the whole world but the realistic pool of alternative suspects, which may be small. Both fallacies misrepresent what the RMP is measuring.
- Field
- Forensic statistics, DNA evidence interpretation
- Core error
- Treating a large population match count as zero evidential weight
- Correct comparator
- Realistic pool of alternative suspects, not the whole population
- Related error
- Prosecutor's fallacy (opposite direction)
Common questions
Why is comparing to the whole world population the wrong approach?+
The relevant question is how many people could realistically have committed the crime, not how many people alive share the profile. If the pool of realistic suspects is a few dozen people connected to the case, a profile shared by millions worldwide can still carry strong evidential weight within that pool.
How do experts avoid both the prosecutor's and defence fallacies?+
Presenting the RMP alongside a clearly stated likelihood ratio, and explicitly defining the relevant population of alternative suspects rather than the general population, keeps the statistic tied to the actual question the court needs answered.
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...
- 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...
- Prior Probability
- The probability of guilt (or innocence) based on all evidence other than the specific forensic match under consideration. Bayes' theorem requires a...
- 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...
- Prejudicial Effect
- The risk that evidence will influence a fact-finder's decision through emotion, bias, or confusion rather than through rational inference. Courts balance prejudicial...
- Probative Value
- The strength of the evidence's tendency to prove or disprove a fact in issue. High probative value means the evidence substantially changes...
- 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...
- 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...
- Reference Class
- The population against which a probability or frequency is calculated. Choosing the wrong reference class, for example using general population allele frequencies...
- Relevance
- The logical quality of evidence that makes a fact in dispute more or less probable. Under FRE Rule 401, evidence is relevant...
- Relevant Population
- The population of individuals who could plausibly have left the forensic sample, constrained by geography, time, and other case facts. Choosing the...
Explained in these topics
- History of Statistical Evidence in CourtsThe mirror error: arguing that because many people in the population share a feature, the evidence against any individual who has that feature is weak. This ig...
- Probative Value and the Weight of EvidenceThe mirror error: arguing that because a large number of people in a population could match the evidence, the evidence has no probative value against the defen...
- The Concept of Random Match Probability
- The Defence FallacyThe erroneous argument that a match probability of one in N is evidentially worthless because a large population of size M would contain M/N potential matches....
- The Prosecutor's FallacyThe mirror-image error in which the rarity of a match is dismissed as meaningless because the absolute number of matching people in the population is large. Bo...