Rarity Estimate
Definition
A numerical statement of how uncommon a particular feature combination is in a reference population. Derived from frequency databases or empirical studies. For non-DNA evidence, rarity estimates are often based on smaller and less representative databases than their DNA counterparts.
- Expresses
- How uncommon a feature combination is
- Source
- Frequency databases or empirical studies
- Limitation
- Non-DNA databases often smaller, less representative
- Applies to
- Non-DNA pattern and trace evidence
Common questions
Why are rarity estimates for non-DNA evidence considered weaker than DNA match probabilities?+
DNA population databases are large, well-characterised, and built on validated allele-frequency models. Comparable databases for shoe patterns, glass composition, or fibre types are often smaller, less systematically sampled, or absent, so the resulting rarity figure carries more uncertainty.
How should a rarity estimate be presented to avoid overstating its strength?+
Good practice is to state the estimate with its source database and its known limitations, and to avoid presenting it as a precise statistical probability when the underlying database cannot support that level of precision.
Related terms
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- PCAST Report
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- Refractive Index (RI)
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