The math behind your paternity test results
On the Beta Paternity website, we state:
“Our testing is so powerful that we can guarantee [a] probability of paternity [of] 99.9999% for all cases in which the mother, alleged father, and child are all tested”
But what does “99.9999% probability of paternity” mean?
Probability of paternity, as reported on paternity test results, represents the chance or certainty that an alleged father is the biological father of a child, as compared to a random untested male in the same population. This number is based on the combined paternity index (CPI), a statistical value reported on DNA test results.
A probability of paternity of 99.9999% means that there is 1,000,000 to 1 certainty that the tested alleged father is the biological father of the child.
To determine the probability of paternity, our DNA analysts must first compare the length of specific genetic markers between the alleged father, the mother, and the child to determine paternity indices (PI). A paternity index is generated for 15 of the 16 standard markers examined (the 16th marker is a gender marker that has no PI). When the child has the same length of genetic marker as the alleged father, the PI will be above 0 (if it is not the same, the PI will be 0). After all of the markers are compared and the PI generated, the CPI is determined by combining these PI.
In trio paternity tests, the CPI is typically 0 for an exclusion or very high (often above 500,000) for an inclusion. The CPI is an important value, as it is converted to determine the probability of paternity.
Specifically:
Probability of Paternity = (100 x (1 – 1/CPI))%
This yields a percentage equal to the likelihood that the alleged father is the biological father of the child.
If you have a CPI of 1,000,000, you can determine that the probability of paternity is 99.9999%. This means that it is 99.9999% likely that the alleged father is the biological father of the child.
Beta is dedicated to using state-of-the-art technology and expert knowledge to provide you with the highest quality DNA testing. This is why we can guarantee such conclusive results. To learn more about our DNA testing services, call one of our testing specialists today at 1-800-798-3810.
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on Thursday, March 8th, 2007 at 4:02 pm and is filed under Home Paternity Tests.
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Digg it
April 18th, 2007 at 2:28 pm
So what does it mean when you hear Larry Birkhead is the father of Dannielynn by 99.99999%? Is that even better than 99.9999%? What’s with all the 9s??
April 20th, 2007 at 3:56 pm
Great questions, Farrah.
Based on the equation for Probability of Paternity we provided in our initial post, you can determine what a probability of 99.99999% would be in terms of odds or CPI.
If you review the interviews with court-appointed DNA expert Dr. Michael Baird , you will read that the 99.99999% probability of paternity for Larry Birkhead as the biological father of Dannielynn was determined based on a CPI of 11,515,619. This means that the odds of Birkhead being the biological father are 11,515,619 to 1.
As for the “9s” after the decimal in probabilities of paternity, these do in fact correlate with the certainty of the test results. The more 9s after the decimal, the more certain you can be that the alleged father is the biological father of the child. Generally, however, you will see results from 99.9% to 99.99999%. Results in these ranges are considered highly certain, accurate, and conclusive.
Thanks for the questions, Farrah. Keep the questions coming!
-Beta moderator