When the estimated frequency of the shared profile is very low, some labs will simply state “to a scientific certainty” that the samples sharing that profile are from the same person. For example, the FBI laboratory will claim two samples are from the same person if the estimated frequency of the shared profile among unrelated individuals is below one in 260 billion. Other labs use different cut off values for making identity claims. All of the cut-off values are arbitrary: there is no scientific reason for setting the cut off at any particular level just as scientists have not arrived at any formally recognized way of being “scientifically certain” about anything (in fact, many would argue that it is essential for scientists to be uncertain about essentially everything). Moreover, these identity claims can be misleading because they imply that there could be no alternative explanation for the “match,” such as laboratory error, and they ignore the fact that close relatives are far more likely to have matching profiles than unrelated individuals. They can also be misleading in that the DNA tests themselves are powerless to provide any insight into the circumstances under which the sample was deposited and are generally unable to determine the type of tissue that was involved.
Budlowe, B., Chakraborty, R., Carmody, G., and Monson, K. (2000). Source attribution of a forensic DNA profile. Forensic Science Communications. 2(3).
Weir, B.S. Are DNA profiles unique?.