To get a more thorough understanding of the results to your DNA test, you need to understand the statistical analysis undertaken by laboratory analysts to come to your results. Once results from all donors for all the DNA markers have been collected, they are input into a statistical software program for interpretation.
For a given marker in a paternity test, if the number of repeats (or frequency) does not match between a child and alleged father, it is highly likely that the latter is not the real biological father. There is however, a slight chance that the discrepancy may be due to other causes, such as a mutation that may have occurred between father and child. So in such cases the conclusion cannot be made with certainty. For each additional locus in which the frequency differs between alleged father and child, there is an increase in the probability that the alleged father is not the biological father. In fact, if the frequencies differ in two or more loci, it is practically impossible that the alleged father is the real biological father.
On the other hand, a match in the frequencies between an alleged father and a child for a given marker may occur for two reasons. It may either be that the alleged father is the real biological father. However it could also be that the two donors happen to have identical frequencies purely by chance. The probability of the latter occurring depends to a large extent on the ethnic origin of the donors. For this reason, population databases defined by ethnic group and geographical region are generally employed to calculate probabilities of a match occurring if the two donors are not related. (This is the reason why laboratories generally request information on the ethnic origins of the donors.) The larger the number of matching markers between a child and an alleged father, the higher the likelihood that the latter is the biological father. For this reason it is better to select tests that employ the standard 16 marker analysis.
During a paternity test, the laboratory analysts examine the markers one by one. Whenever a match occurs between the child and alleged father, the frequency observed for the particular locus is looked up in a population database, and the probability value obtained is input in the statistical program. The program processes the information and outputs a result called a paternity index for this particular locus. When all loci are analysed and input, the program computes the combined paternity index and the probability of paternity for the overall test.
Two important terms in your Paternity Test Result
The combined paternity index (CPI) is a likelihood ratio that expresses how many more times an individual is likely to be the biological parent as compared to an unrelated and untested person of the same ethnic group. An index of 100 or greater is considered strong genetic evidence of parentage.
The probability of paternity is defined as a percentage, and it compares the likelihood that the tested individual has passed on the particular marker frequencies to the likelihood that an untested random person of the same ethnic group may have passed on these frequencies. If an alleged father is not the biological father, the paternity test generates a probability of paternity of 0%. If the alleged father is the biological father, the test usually generates a probability of paternity on the order of 99.99% or more.
Proceed to the following link for information about the Paternity Test Result format and examples.
