Thursday, October 22, 2015

SWGDAM Guidelines on "Probabilistic Genotyping Systems" (Part 1)

In June, the Scientific Working Group on DNA Analysis Methods (SWGDAM), approved new “Guidelines for the Validation of Probabilistic Genotyping Systems.” 1/ They begin,
Guidance is provided herein for the validation of probabilistic genotyping software used for the analysis of autosomal short tandem repeat (STR) typing results. These guidelines are not intended to be applied retroactively. It is anticipated that they will evolve with future developments in probabilistic genotyping systems.
These three sentences, raise four questions. First, is the phrase “probabilistic genotyping system” (PGS) the best label? I will get to the question of what “probabilistic” means a little later, but given the perception of segments of the public and the legal community that “autosomal short tandem repeat (STR) results” are “very likely” “to reveal predispositions to diseases in the individuals being profiled as well as their siblings and offspring,” 2/ is “genotyping” the right word to use for identifying DNA variations that are not genes? A more neutral term such as “probabilistic typing systems” might be less suggestive.

Second, why do the drafters of standards and guidelines prefer stilted writing—“guidance is provided herein”—as opposed to plain English sentences such as “This document offers guidance”? I know this kind of criticism is small potatoes, but scientists are smart enough to be good writers.

Third, what are the drafters trying to say with the doubly passively voiced sentence, “These guidelines are not intended to be applied retroactively”? Who should not apply these standards retroactively? One would think that the guidelines are for laboratories, but how could a laboratory apply a recommendation retroactively? It cannot go back in time to validate software that it has been using even though neither it nor the developer had validated the software in the manner that SWGDAM now recommends. The only thing the laboratory could do to give retroactive effect to the new advice would be to use some better validated software on data from old cases and advise prosecutors, defendants, or defense lawyers of major discrepancies. Is SWGDAM saying that looking back at past cases (for research or other purposes) would be wrong? Or merely that SWGDAM is taking no position on the desirability of undertaking such retrospective analyses? Or is this part of the guidelines written for a difference audience—courts that might be asked to grant postconviction relief? But unless every PGS was adequately validated, surely courts should consider what these guidelines have to say as relevant to (but not necessarily dispositive of) whether the laboratory’s earlier report was scientifically acceptable. Most courts can be expected to appreciate the fallacy of the argument that "because the world gets wiser as it gets older, therefore it was foolish before." 3/

Fourth, why does SWGDAM anticipate that “future developments in probabilistic genotyping systems” will cause these standards to “evolve”? The principles of good software development and validation do not depend on the specific programs. Those principles may evolve whether or not PGSs improve over time. Of course, the guidelines could change if the programs become so superior that SWGDAM would reconsider its view (expressed in the next paragraph) that the only permissible use of a PGS is “to assist the DNA analyst in the interpretation of forensic DNA typing results.” Is SWGDAM envisioning that it could reverse its opinion that “Probabilistic genotyping is not intended to replace the human evaluation of the forensic DNA typing results” because of “future developments in [PGS]”? In light of current problems with human interpretations of mixtures of minute quantities, there are observers who would welcome replacing the current protocols for interpreting these samples with valid and reliable automated expert or probabilistic systems.


1. Scientific Working Group on DNA Analysis Methods, Guidelines for the Validation of Probabilistic Genotyping Systems, June 15, 2015

2. Gary R. Skusea1 & Anne M. Burgera, Justice as Fairness: Forensic Implications of DNA and Privacy, Champion, Apr. 2015, at 24. For a more authoritative assessment, see Henry T. Greely & David H. Kaye, A Brief of Genetics, Genomics and Forensic Science Researchers in Maryland v. King, 53 Jurimetrics J. 43 (2013).

3. Hart v. Lancashire &Yorkshire Ry. Co., 21 L.T.R. N.S. 261, 263 (1869).

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