Theta Correction (FST or Coancestry Coefficient)
Definition
A statistical adjustment applied to DNA match probability calculations to account for expected allele-sharing within subpopulations due to substructure. A theta value of 0.01 to 0.03 is commonly applied in forensic practice; higher values are more conservative. Also called the coancestry coefficient or FST.
- Purpose
- Adjust match probability for subpopulation structure
- Typical value
- 0.01 to 0.03 in forensic practice
- Effect of higher theta
- More conservative match probability
- Also called
- FST or coancestry coefficient
Common questions
Why is a theta correction needed at all?+
Members of the same subpopulation share alleles more often than random members of the full reference database, so ignoring this structure can overstate the rarity of a matching DNA profile.
Does a higher theta value help or hurt the defence?+
A higher theta produces a more conservative, lower estimate of how rare a matching profile is, which generally favours the defendant by reducing the reported statistical weight of the match.
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...
- Minimum Allele Frequency (Minimum Frequency Ceiling)
- A conservative lower bound applied when an allele is absent or very rare in the database, to prevent an artificially inflated match...
- 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...