Minimum Allele Frequency (Minimum Frequency Ceiling)
Definition
A conservative lower bound applied when an allele is absent or very rare in the database, to prevent an artificially inflated match probability statistic. Common values are 5/2N (where N is the database size) or a fixed floor such as 0.001 (one in a thousand).
- Common formula
- 5/2N, where N is the database sample size
- Typical fixed floor
- 0.001 (one in a thousand)
- Purpose
- Prevents an artificially inflated match probability
- Applies to
- Alleles absent or very rare in the reference database
Common questions
Why not just use the observed frequency of zero for an unseen allele?+
A zero or near-zero observed frequency would make the calculated match probability implausibly small, overstating the strength of the DNA evidence. The ceiling substitutes a conservative, defensible minimum so the statistic cannot be inflated by database sampling limits.
Does the ceiling favour the prosecution or the defence?+
It favours the defence. Using a floor larger than the true, unknown rare-allele frequency makes the reported match probability more conservative, that is, less impressive, than an uncapped calculation might produce.
Is the ceiling applied to every allele in a profile?+
No, only to alleles that are absent or observed at very low counts in the relevant population database. Common alleles use their directly observed database frequency without adjustment.
Related terms
- Allele Frequency
- The proportion of a specific allele variant at a given genetic locus in a defined population. Forensic match probability calculations multiply the...
- 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...
- Population Substructure
- The condition in which allele frequencies differ systematically between subgroups within a nominally single population. Substructure means that individuals within a subgroup...
- Reference Population Database
- A curated collection of allele frequencies or feature measurements from a defined population, used to estimate the probability that a randomly chosen...
- Theta Correction (FST or Coancestry Coefficient)
- A statistical adjustment applied to DNA match probability calculations to account for expected allele-sharing within subpopulations due to substructure. A theta value...