How accurate is a probability of paternity of 99.9%?
DNA paternity testing is a very powerful form of testing. In practice the DNA test results are often expressed in terms of the probability of paternity. In the event of a likelihood ratio of 99.9%, German courts consider paternity to be "practically proven". However, the probability of paternity statistic can be intuitively misleading.
What does a probability of 99.9% really mean?
Many paternity testing laboratories guarantee their test results to an accuracy of 99.9%. Although this sounds extremely accurate, the number is misleading. A 99.9% "match" means that the test has identified a genetic pattern that is possessed, on average, by one in every 1000 men in the population.
For example: In a stadium with 50000 random men, 50 of these men could not be excluded from the possibility of being the father by the DNA test.
In our opinion, a probability of paternity of 99.9% is not sufficient to be certain of paternity. A 12 marker STR test is usually not discriminating enough to provide conclusive results. By analysis of far more than the standard 12 markers paternity probabilities up to 99.99999999999999% can be obtained.