Theta (FST) Correction
Definition
A correction factor applied to allele frequency estimates to account for population sub-structure (the tendency for allele frequencies to differ between sub-populations). Using theta inflates the RMP slightly, making the statistic conservative in favour of the defendant. Typical values range from 0.01 to 0.03 for well-characterised populations.
- Corrects for
- Population sub-structure between sub-populations
- Effect on RMP
- Inflates the statistic, favouring the defendant
- Typical range
- 0.01 to 0.03 for well-characterised populations
- Applied alongside
- Product rule calculations
Common questions
Why does a theta correction make the RMP more conservative rather than less?+
Population sub-structure means allele frequencies can differ between sub-populations within a broader group, so ignoring it risks underestimating how common a genotype actually is. Applying theta raises the calculated RMP, which is the conservative, defence-favourable direction.
Does a higher theta value always apply to more genetically diverse populations?+
Not exactly. Theta reflects the degree of sub-structure or relatedness within the specific reference population used, which can be influenced by factors such as founder effects or limited intermarriage, not simply overall genetic diversity.
Who decides which theta value to use in a given case?+
Laboratories typically follow published values for well-studied population groups relevant to the case, chosen based on the reference database used for the calculation, and the specific value applied is disclosed in the laboratory report for scrutiny.
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...
- 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...
- 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...
- 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...
- Defence Attorney's Fallacy
- The incorrect argument that a large global or national population means many unrelated individuals could share the profile, while ignoring the geographic...
- Defence Fallacy
- The converse error of inflating the importance of the RMP by arguing that, because many people in the population share the profile,...
- ENFSI DNA Working Group
- The European Network of Forensic Science Institutes DNA Working Group. Coordinates European forensic DNA standards, including the evaluative reporting guidelines adopted by...
- Prior Odds
- The ratio of the probability of the prosecution hypothesis to the probability of the defence hypothesis before the forensic evidence in question...
- SWGDAM
- Scientific Working Group for DNA Analysis Methods; the US body that publishes interpretation guidelines and validation requirements for forensic DNA laboratories operating...
Explained in these topics
- DNA Match Probability CalculationAn adjustment recommended by NRC-II for population substructure. For a heterozygote with alleles a and b from subpopulation with allele frequencies p and q, th...
- Allele Frequencies, Random Match Probability and the Likelihood RatioA correction factor applied to HWE genotype frequency calculations to account for population substructure (non-random mating within subpopulations). Typically...
- The Concept of Random Match Probability