RESULTS & INTERPRETATION OF DNA EVIDENCE
Laboratory analysis of questioned and known samples will produce three possible results: an inclusion, an exclusion, or inconclusive.
An inclusion indicates a direct association exists between the questioned and known samples. Typically referred to as a match, this association really means one of three possibilities:
- The samples originated from a common source. Thus it can be concluded the evidence recovered from a crime scene originated from the provider of the known sample.
- Similarity in the sample is a coincidence. The questioned sample originates from someone other than the subject who provided the known sample, who happens to have the same profile. Thus the sample provided from the known subject and the true depositor share the same genetic profile.
- The similarity is an accident. Due to collection or analytical error, the questioned and known samples appear to have the same DNA profile. The possibility of sample mix-up or laboratory error will be minimal if lab protocols follow standard SWGDAM guidelines.
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Should comparison of the questioned and known samples indicate a non-match, the result will be expressed as an exclusion. This means that the two samples do not originate from the same source, or that two individuals are unrelated in the case of reverse paternity-type analysis. This exclusionary determination is perhaps one of the more powerful aspects of DNA analysis as it is absolute. As has been previously illustrated, the results of DNA analysis have resulted in the exclusion of potential suspects and the release of persons wrongly convicted.
Results of analysis may be deemed inconclusive for a number of reasons. Either the questioned or known sample is degraded, is of insufficient quantity, or problems have occurred in the laboratory processing. Note, this does not mean a potential suspect is eliminated. It simply means the results of DNA analysis are of no evidentiary value. If the inconclusive result is from questioned samples found at the scene, then it may be that these DNA samples have no evidentiary value. This underscores the need to ensure samples are collected, transported, preserved, and stored according to established procedures. An inconclusive result may also require the investigator to apply for a new warrant, which may not be granted.
The investigative value of forensic DNA analysis is predicated upon the assumption that a particular subject or suspect is the source of biological evidence recovered from a crime scene. Establishing an association (inclusion) between the questioned and known samples in and of itself is insufficient. The real significance of the association is determined by the statistical interpretation provided by the lab. The role of the lab is not to tell police that the person who is the subject of the DNA analysis is the suspect. Rather, it is to make a statement that the possibility of someone else being the provider of the sample is so unlikely as to not be credible. “The strength of a match as an absolute identifier is dependent on how common or rare the matching portion of the questioned and known samples are in the [relevant] population.” This relates to the probability that the match is a coincidence. The more common the questioned DNA profile is in the population, the more probable it becomes that someone other than the suspect could have left the crime scene sample.