Base-Rate Fallacy
Definition
Ignoring the background frequency (base rate) of a characteristic when interpreting a match. A test specific to 1 in 10,000 people still flags many innocent individuals in a large population, so the absolute number of potential matches must be considered alongside the match probability.
- Related concept
- Prosecutor's fallacy
- Error type
- Ignoring background frequency of a trait
- Example test specificity
- 1 in 10,000
- Consequence
- Many false matches in a large population
Common questions
How does the base-rate fallacy show up in courtroom testimony?+
It appears when a witness states a match probability like one in ten thousand and lets the jury infer near-certain guilt, without mentioning that in a database or population of millions, that same probability implies thousands of other people would also match. The rarity of a match and the number of people it could still apply to are different questions.
What is the practical fix for avoiding the base-rate fallacy in reporting?+
Reporting should state both the match statistic and, where relevant, the size of the population being considered, so the fact-finder can see how many innocent matches the statistic would be expected to produce, rather than treating rarity alone as proof of identity.
Related terms
- Error Rate
- One of the Daubert factors: the known or potential rate at which a method produces false positives or false negatives, and whether...
- 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...
- Measurement Uncertainty
- A quantitative statement of the range within which the true value of a measurement lies, at a stated confidence level (typically 95%,...
- Probabilistic Reporting
- The practice of stating a dental age estimate as a range with an associated probability (e.g. a 95% confidence interval) rather than...
- 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...