Statistical Interpretation of the Match
Many courts require statistical interpretation of a DNA match. Random match probabilities are most often used to interpret evidence from single source samples.
Generally, once a DNA match is determined, a statistical computation is performed to estimate how often a random unrelated person would be found with that particular DNA profile.
Statistical Calculations
Once a DNA match is observed, forensic scientists estimate the chance of finding that DNA profile in particular human populations. This calculation is necessary to inform the jury of the rarity of the profile.
Individuals can often be distinguished from one another at even a single locus because 8-20 alleles have been observed in representative populations at each of the 13 STR loci.
Statistical Calculations and Source Attribution
Once an individual's 13-locus STR profile is identified, it is statistically improbable that anyone else in the world will have the same profile, unless that person has an identical twin. Identical twins (twins derived from a single fertilized egg) have identical STR DNA profiles.
Based on individual laboratory policy, the report may include a statement of source attribution in addition to, or in place of, a statistical estimate.
Probability
The term "probability" refers to the chance of a particular event occurring. For example, the probability of observing a head in a coin toss would be one out of two, or one-half.
Probability and Population Databases
Population databases of major racial and ethnic groups are used to determine estimates of the rarity of DNA profiles. These databases sometimes consist of as few as 100 profiles from unrelated persons, yet allow a reliable estimate of the chance of observing a given DNA profile in a larger population.
The content on this page is taken from Statistics and Population Genetics module of the free, self-paced, online training course Forensic DNA for Officers of the Court.
