Combined Paternity Index (CPI)
Definition
The product of all individual paternity indices across all marker systems tested. Assumes independence of the marker systems. The CPI is a likelihood ratio; converting it to a probability requires a prior probability.
- Calculation method
- Product of per-locus paternity index values across all autosomal STR loci
- Standard use
- Summary statistic in DNA paternity reports
- Conversion metric
- Can be converted to Probability of Paternity (POP) using Bayes' theorem
Common questions
What is a Combined Paternity Index and how is it calculated?+
A Combined Paternity Index (CPI) is the product of individual paternity index values from all autosomal STR loci tested in a DNA analysis. In other words, you multiply the PI values across all the genetic markers to get one summary number that represents the overall likelihood.
How is CPI used in paternity cases?+
The CPI is the standard summary statistic reported in DNA paternity cases. It can be converted to a Probability of Paternity (POP) using Bayes' theorem with an equal prior, which gives the court or laboratory a single quantified measure of likelihood.
Why multiply individual locus values instead of adding them?+
Multiplication combines the discriminating power of independent genetic markers. Each locus contributes a separate probability ratio, and multiplying them together gives a cumulative strength of evidence across all tested locations in the DNA.
Related terms
- Paternity Index (PI)
- A likelihood ratio that compares the probability of the observed serological profile if the alleged father is the biological father to the...
- 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....
- 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
- The categorical conclusion that one or more examined features are clearly inconsistent in a way that cannot be explained by projection angle,...
- 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...
- Familias
- Open-source kinship LR software developed in Norway, the reference platform for ISFG kinship workshops and used operationally by forensic labs in Sweden,...
- Inbreeding Coefficient (F)
- The probability that two alleles at a locus in an individual are identical by descent, reflecting population consanguinity. Non-zero F values affect...
- Inclusion
- All obligate alleles are accounted for. Inclusion is probabilistic, never absolute, and is reported as a CPI plus probability of paternity.
- Motherless Paternity Test
- Paternity analysis where the mother's sample is unavailable. CPI thresholds are raised because the obligate paternal allele cannot be inferred from the...
- Mutation Rate
- STR loci mutate at roughly 0.001 to 0.005 per generation. A single one-step mismatch is typically scored as a possible mutation, not...
Explained in these topics
- Disputed Paternity and Maternity: Forensic Testing in IndiaProduct of the per-locus PI values across all autosomal STR loci tested. The standard summary statistic in an Indian DNA paternity report.
- Paternity, Complex Kinship and Missing-Persons CaseworkThe product of individual-locus paternity indices across all typed loci. Converted to Probability of Paternity (POP) using Bayes' theorem with an equal prior.
- Statistical Interpretation of Serological Relationship EvidenceThe product of all individual paternity indices across all marker systems tested. Assumes independence of the marker systems. The CPI is a likelihood ratio; co...