Statistics Behind a Match
To find a match, forensic analysts compare the genetic profile obtained from crime scene evidence to the profile from a known individual (e.g., suspect, victim).
If the DNA profiles from the evidentiary and known samples are the same at each locus, laboratory analysts can provide a determination of the statistical significance of the evidence. In some cases, no conclusive interpretation can be made.
Typically there are three possible laboratory outcomes:
- If the DNA profiles from the evidentiary and known samples are consistent at each locus, laboratory analysts can interpret this finding as a "match", "inclusion", or "failure to exclude."
- If the two profiles are not consistent at each locus, the finding can be interpreted as a "nonmatch" or "exclusion."
- If there are insufficient data to support a conclusion, the finding is often referred to as "inconclusive."
In a given population, any particular 13-locus short tandem repeat (STR) profile is rare because there are many distinct alleles at each of these STR loci. Therefore, it is possible to distinguish between individuals.
Practical issues (e.g., small amounts of DNA) can limit the ability to obtain results; additionally, not all laboratories routinely type the 13 STR loci required for the national DNA database. Nonetheless, it is still possible to distinguish between individuals.
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.
