Paternity Index (PI)
Definition
A likelihood ratio that compares the probability of the observed serological profile if the alleged father is the biological father to the probability if a random man from the population were the father. A PI greater than 1 favours the alleged father; values below 1 are rare in non-exclusion cases and indicate a serological finding more common in unrelated men.
- What it is
- A likelihood ratio calculated at each DNA locus, comparing two conditional probabilities.
- Computation scope
- Calculated per STR locus and combined across multiple loci to produce the Combined Paternity Index (CPI).
- Interpretation
- Higher ratios indicate stronger evidence the alleged father is the biological father. Lower ratios suggest he is not.
Common questions
What does a paternity index actually tell you?+
It's a number that compares the odds the tested man is the father versus the odds a random man from the population would match. A higher number means stronger evidence of paternity. Multiple loci are combined into a Combined Paternity Index to give the overall likelihood.
How is the PI calculated?+
At each DNA locus, it's the probability of the child's alleles given the alleged father is the biological parent, divided by the probability if an unrelated man from the same population were the father. This ratio is then combined across all tested loci for the final result.
Can a low PI rule out paternity?+
Yes. A PI less than 1 means the evidence favors the tested man not being the father. Different thresholds are used by labs and courts to determine conclusive exclusion or inclusion, but the PI provides the statistical basis for that decision.
Related terms
- Combined Paternity Index (CPI)
- The product of all individual paternity indices across all marker systems tested. Assumes independence of the marker systems. The CPI is a...
- Exclusion
- The categorical conclusion that one or more examined features are clearly inconsistent in a way that cannot be explained by projection angle,...
- Probability of Paternity (W)
- The posterior probability that the alleged father is the biological father, calculated from the CPI and a prior probability using Bayes' theorem....
- Complement-Dependent Cytotoxicity (CDC)
- A serological HLA typing method in which test lymphocytes are incubated with HLA-specific antibody and then with complement. If the antibody binds,...
- Deficient Paternity Test
- Paternity inferred from the alleged father's relatives (grandparents, siblings) when the alleged father himself is unavailable or deceased.
- Duo / Motherless Case
- A paternity test in which the mother's profile is unavailable; the PI denominator must integrate over all possible maternal allele contributions from...
- Essen-Moller W
- The formula for probability of paternity derived by Erik Essen-Moller in 1938 and widely adopted by forensic laboratories. W = CPI /...
- Exclusion Probability (PE)
- The proportion of unrelated men in the relevant population who would be excluded as fathers of a child with the observed maternal...
- Exclusion Probability (W)
- The proportion of the male reference population that would be excluded as the biological father by the marker panel tested. A high...
- False Exclusion
- An incorrect serological finding that excludes a true biological parent. Causes include chimerism (two cell populations in one individual), rare antigen variants,...
- Familias
- Open-source kinship LR software developed in Norway, the reference platform for ISFG kinship workshops and used operationally by forensic labs in Sweden,...
- HLA Haplotype
- The specific combination of HLA alleles inherited as a block on one copy of chromosome 6. Because HLA loci are tightly linked,...
Explained in these topics
- Blood Group Typing in Paternity and Kinship TestingA likelihood ratio that compares the probability of the observed serological profile if the alleged father is the biological father to the probability if a ran...
- Disputed Paternity and Maternity: Forensic Testing in IndiaLikelihood ratio for a single locus: probability of the observed child-father genotype combination if the alleged father is the true father, divided by the pro...
- The HLA System and Its Role in Forensic Relationship TestingA likelihood ratio comparing the probability that the alleged father transmitted the obligate paternal allele(s) to the probability that a random man from the...
- Paternity, Complex Kinship and Missing-Persons CaseworkThe likelihood ratio at a single STR locus comparing the probability of the child's allele given the alleged father is the biological father versus a random ma...
- Statistical Interpretation of Serological Relationship EvidenceA likelihood ratio for a single genetic marker: the probability of the child's genotype if the alleged father is the true father, divided by the probability if...