How can an analyst translate uncertain DNA data into understandable testimony for a non-expert jury?
A DNA analyst’s life alternates between laboratory routine and courtroom unpredictability. In the lab, biological evidence is chemically separated into DNA data. Simple evidence (such as one man’s blood on a knife) produces simple data, easily reported and explained in court. The greater complexity of today’s low-level and mixed DNA introduces interpretation and courtroom complications, however. How can the analyst translate uncertain DNA data into understandable testimony for a non-expert jury? How well will evidence and analyst hold up under vigorous cross-examination?
The DNA match statistic is a powerful tool for taming DNA uncertainty. The statistic, or “likelihood ratio,” summarizes in a single number the evidential support for a person having contributed DNA. With simple evidence from a single person, this ratio is easily explained:
- the numerator (upper half) is “1,” the chance of a match if the prosecutor is correct, while
- the denominator (lower half) is the prevalence of the person’s genotype in the population, i.e., the chance of a coincidental match when the prosecutor is mistaken.
Multiplication combines the statistics from all independent genetic loci. We can then read a one in quadrillion (a one followed by 15 zeros) genotype rarity as the understandable statement: “a match between evidence and suspect is a quadrillion times more probable than coincidence.”
What about DNA mixtures? Alas, those are not so simple. The analyst must explain to a jury how a “threshold” classified data peaks into “alleles,” but also that this threshold varies between laboratories and may give conflicting or inaccurate results. Scientific certainty devolves into subjective opinion, susceptible to cross-examination scrutiny. How can the analyst reestablish the comfort of single-source samples when testifying about mixtures in court? Without solid scientific assurance, forensic laboratory policy may discard informative crime-fighting DNA evidence as “inconclusive,” simply because the interpretation or courtroom presentation is too hard for a human analyst.
Fortunately, modern computing can help. An objective probabilistic computer can thoroughly examine the DNA mixture evidence, separating out the genotypes of each person who contributed. A separated contributor genotype can then be matched and explained with the ease and simplicity of single-source DNA. With this computer assistance, an analyst can confidently testify in court about objective scientific mixture results, without branding vital evidence “inconclusive.”