The Combined Paternity Index (CPI) is a statistical measure used in paternity testing to quantify the likelihood that a specific individual is the biological father of a child. It provides a numerical representation of genetic evidence, helping to differentiate between an alleged father and any randomly selected individual. The CPI ultimately helps to formulate the probability of paternity, expressed as a percentage.
What Paternity Index Means
Paternity testing analyzes specific genetic markers, which are distinct DNA locations. For each marker, a Paternity Index (PI) is calculated. This PI is a statistical ratio comparing how much more likely the alleged father transmitted the marker to the child than a randomly selected, unrelated man from the same ethnic group.
For example, if an alleged father and child share an allele, the PI assesses the match’s strength based on the allele’s population frequency. A rare shared marker yields a higher PI, indicating stronger evidence. The PI for each marker measures its statistical weight in supporting or refuting parentage and is a likelihood ratio.
How Combined Paternity Index is Calculated
Individual Paternity Indices (PIs) for multiple genetic markers are multiplied to calculate the Combined Paternity Index (CPI). This multiplicative effect means more markers analyzed lead to a higher CPI. Multiplying PIs across various genetic loci significantly increases the test’s statistical power.
For instance, if each of 20 loci has a PI of 10, the CPI would be 10 raised to the power of 20, resulting in an exceptionally large number. This process leverages the uniqueness of an individual’s DNA profile, creating a robust statistical measure of paternity. The CPI measures the strength of genetic evidence and is an odds ratio, not a direct probability.
Interpreting High CPI Values
A high Combined Paternity Index (CPI) signifies a strong statistical likelihood that the alleged father is the biological father. While no theoretical “highest” limit exists, CPI values can reach into the hundreds of thousands, millions, or billions. These high numbers translate into an overwhelming probability of paternity, typically 99.9% or higher. For example, a CPI greater than 1000 usually indicates over 99% probability.
Such values mean the tested man is many times more likely to be the biological father than a randomly selected, unrelated man, effectively confirming paternity for practical and legal purposes. Legal standards often require a probability of paternity of 99.0% or higher, with some cases, like immigration, requiring even higher thresholds, such as a CPI of at least 200 (99.5% probability).
Factors Influencing CPI
Several factors contribute to the Combined Paternity Index (CPI). The number of genetic markers analyzed directly impacts the CPI; more markers generally lead to a higher index. DNA paternity tests typically examine 15 to 20 or more Short Tandem Repeat (STR) markers.
The uniqueness or rarity of shared genetic markers also plays a role. Rarer alleles, less frequent in the general population, contribute more substantially to the CPI, as a match provides stronger evidence. The alleged father’s genetic relationship to the child can influence interpretation; if a close relative could also be the father, additional analysis may be needed.
The quality and completeness of DNA samples are crucial. Degraded or contaminated samples can yield inconclusive results or lower CPI values. Laboratories aim for high-quality DNA to ensure reliable outcomes. Including the biological mother’s DNA can also strengthen results, especially when common alleles are shared.