15.4: Absolutes, Probabilities, and Tolerances

Absolutes, probabilities and tolerances are concepts that do appear in forensic analytical reports with some careful consideration. Forensic analysis reports avoid the use of absolutes. Several decades ago, it was not uncommon for a fingerprint comparison report to include the language that the comparison was a match to the exclusion of all others. The problem with a statement like this is that although in all our efforts we have yet to find two fingerprint patterns that are in agreement, we have yet to record the fingerprint impressions of several generations of everyone in the world. What we can say is that finding two people with the same fingerprint patterns living in the same area to be highly unlikely. Forensic analytical reports also tend to limit the use of probabilistic deductions unless where necessary. Therefore, rather than writing that the estimated arc of fire for the position of a shooter based on the convergence of trajectory and the cluster pattern of fired cartridge casings in the scene is a zone of probability, it is better to say that it is a zone of possibility. Probability would require a certain amount of mathematical conclusions, and there are too many variables involved in a shooting scene investigation. Probabilities are a mathematical method of quantifying the uncertainty of an event of evidence, used to make a determination of the likelihood of an event. Likelihood Ratios compare the probability of the evidence under at least two different hypotheses (i.e. a specific person versus a random person). This is contrasted by the probability that a random person would match the evidence. This is called the Random Match Probability standard. Probability is important in DNA analysis because the Random Match Probability includes the possibility of a coincidental match, but the Likelihood Ratios compare the strength of the evidence for a person being the source of the match as opposed to a random person being the contributor of the source. Therefore, the Random Match Probability of the match between the contributor of the source DNA and an unrelated person can be extremely, well, unlikely. In fact, it can be as low as one in a billion people. Tolerance can appear in two ways, both of which can present its own problems. In forensic science, tolerance refers to margins of error in analysis. If you recall following the Daubert Decision that the judge is the gatekeeper of the expert witness testimony and one of the considerations for the testimony to be allowed is the known error rates. Error rates may be known in scientific experimentation, but not necessarily in the forensic sciences where observations and opinions may not have known error rates. Therefore, it is important that the forensic expert must be aware that their analysis may contain errors. Consider that a forensic analyst compared the firing pin marks of fired cartridge casings from a shooting scene with fired cartridge casings recovered from a firearm suspected of having been used in the shooting. The casings appear to be in agreement. The analyst would be cognizant that they had not compared every firearm in the country and therefore would be in error if they reported that the firearm was the definitive contributor of the fired cartridge casings found in the shooting scene. The analyst, however, would not be incorrect to report that the fired cartridge casings were in agreement and consistent with having been produced by a common source. If asked during testimony if this is a definitive match, the analyst would respond accordingly as not to give the impression that there may not be another firearm in the world that could have fired the cartridge casings found in the scene. Tolerances may also find themselves introduced into courtroom testimony when a forensic technician is testifying about the confirmatory analysis of a drug. This testimony may include the weight and purity of the drug. It is a defense strategy to introduce the concept of drug use tolerance. Tolerance, in this regard, is the diminished response to a specific drug based on its repeated use and the body’s ability to adapt to its continued presence. In other words, it takes more of the drug to achieve a desired result if the person uses the same drug repeatedly.

The analyst should avoid this line of questioning and instead provide an opinion if the quantity of drugs that were analyzed were of a usable amount regardless of their effect on the user. Most states provide statutes for the definition of a usable about as any amount that can be manipulated and consumed.