Tag Archives: low template

Are Two Heads Better than One? Mathematics and the Amanda Knox Case

In yesterday’s New York Times, “Leila Schneps, a mathematician and mystery writer, and her daughter Coralie Colmez” wrote an op-ed entitled “Justice Flunks Math.” They gave this failing grade to an Italian judge who declined to order a second DNA test in the notorious murder case against the American student, Amanda Knox and her Italian ex-boyfriend, Raffaele Sollecito.

If only the judge understood probability theory, they suggest, he might have ruled in favor of the prosecution’s request for another DNA test. As explained in Forensic, Science, Statistics, and the Law, I am not sure that their criticism of “bad judicial math” in this case computes.

Disagreement on What to Call DNA Profiling with Really Small Samples Confuses Courts

In most U.S. jurisdictions, expert evidence derived from a scientific method that is not generally accepted (the “Frye standard”) or adequately validated (the “Daubert standard”) is inadmissible. Applying these standards properly requires distinguishing between a methodology and its application on a particular occasion.1/ An example of confusion over the methodology-conclusion distinction in forensic DNA testing involves what has been called Low Template (LT), Low Copy Number (LCN), or just “touch” DNA.

Unfortunately, scientists do not seem to agree on which term applies to what,2/ muddying the waters for the courts now being asked to rule on the admissibility of LT DNA profiling. The LT DNA profiling procedure amplifies DNA fragments from the standard STR loci, but the samples of DNA are really tiny. In criminal cases, they come from minute deposits of skin cells transferred by handling a gun, gripping a steering wheel, turning a doorknob, throwing a brick, or the like. With suitable techniques for extraction and amplification, STR profiles can be coaxed from as few as 5 to 20 cells. The most obvious strategy is to increase the number of PCR amplification cycles. The danger is that chance effects might result in one STR fragment being amplified much more than another. Parts of the full profile then could drop out, small peaks from unusual fragments at other loci might “drop in,” and a bit of extraneous DNA could contribute to the profile. Other protocols have been proposed for typing such small quantities of DNA.3/

In People v. Megnath, 898 N.Y.S.2d 408 (N.Y. Sup. Ct. 2010), the first published opinion on the need for strict scrutiny of “LCN DNA analysis,” id. at 410, a New York trial court wrote that because

the LCN DNA method of testing as performed by the OCME [Office of the Chief Medical Examiner] is basically the same technique as HCN [high copy number] DNA testing, with the exception of its increased amplification cycles, the Court finds that LCN DNA testing as performed by the OCME is not a novel scientific technique for the purposes of the Frye inquiry. Indeed, both the LCN and HCN forms of DNA testing require the same steps to be taken. These steps, namely extraction, quantitation, amplification, and electrophoresis are virtually identical in both HCN and LCN DNA testing.

Id. at 415. On this basis, this court concluded that rigorous review of the state of the science was unnecessary.

However, inquiring into the extent to which the variations on the usual extraction and amplification procedures represent the same method or a different one misses a more fundamental point. PCR-based STR profiling with capillary electrophoreses is generally accepted for the purpose of producing identifying profiles because experiments have demonstrated its validity and it fits into well established theories of chemistry and biology. It satisfies the validity standard of Daubert for the same reasons. But this foundation might not extend to the domain of the smaller samples. A light microscope works wells for studying bacteria but its magnification is not adequate for viewing much smaller viruses. For that purpose, an electron microscope is required. Radar can track airplanes or flocks of birds, but the signal-to-noise ratio is too low for it to be useful in tracking the flight path of a solitary, high flying butterfly. The radar equipment is identical, and operator is no less skilled at interpreting what he sees on the screen, but the procedure has not been validated (and would not be valid) for butterfly tracking.

Likewise, in the case of touch DNA, the relevant question is not whether the instrumentation and chemicals are identical or whether the analysts are using the same standards for interpretation. It is whether the system has been validated in the range in which it is being used. This is not a question about how well a validated or generally procedure worked on a particular occasion–an issue to be resolved on a case-by-case basis. It is a broadly applicable question about the ability, under the best of conditions, of the equipment and its operators to pick out a weak but true signal from the noise. Until this trans-case question is resolved, admissibility is unjustified under Frye and Daubert.

Fortunately, in Megnath itself, the court conducted a pretrial Frye hearing and found the modified procedure to be generally accepted as valid for touch DNA. Id. But in United States v. Davis, 602 F. Supp.2d 658 (D.Md. 2009), the government, responding to defendant’s effort to dismiss profiling of low template DNA as “the latest fad in DNA testing,” id. at 667, fought to avoid a Daubert hearing by claiming that the laboratory was using the same equipment and interpretive methods that previously had been held admissible as applied to large samples of DNA. The government expert maintained that if no additional cycles were added to the PCR step (and if no other modifications were made in the usual steps), then no LCN testing was occurring. Id. at 669.4/ The defense expert, Dan Krane, responded by defining “LCN testing” so as to depend on the state of mind of the laboratory technician5/ or on the quantity of DNA at the point of PCR amplification.6/ The district court avoided “making a finding with regard to the dueling definitions of LCN testing advocated by the parties” by finding that “the amount of DNA present in the evidentiary samples tested in this case” was in the normal range.7/

This semantic battle over “the proper, scientifically accepted definition of low copy number testing,” Davis, 602 F. Supp.2d at 667, is pointless. It makes no difference whether STR profiling of very small samples is denominated “LCN testing,” “HCN testing,” “STR profiling,” or any other concatenation of letters. The issue is how well the procedure, whatever its called, works with samples that are so small that the stochastic effects in PCR amplification could produce a false profile. There is no single sample size at which excessive amplification of the wrong DNA fragments and the absence of amplification of the right fragments occur. Ever since PCR-based systems were introduced, critics of DNA testing have argued that such DNA typing is subject to these problems. See David H. Kaye, The Double Helix and the Law of Evidence 182-83 (2010). It is not a reason to exclude all PCR-based analyses, but the proponent of the evidence must be able to show that in the relevant sample size range, it is possible to ascertain the signal reliably. This is the trans-case, scientific question of methodology to be answered under Frye or Daubert.


These remarks are adapted from a discussion in The New Wigmore, A Treatise on Evidence: Expert Evidence (David H. Kaye, David E. Bernstein & Jennifer L. Mnookin eds., 2d ed. 2011) (in press).

1. See The New Wigmore, A Treatise on Evidence: Expert Evidence ch. 7 (David H. Kaye, David E. Bernstein & Jennifer L. Mnookin eds., 2004).

2. Whether the terms are truly synonymous is itself the subject of contention. See, e.g., Bode Technology, Touch DNA–Overview, http://www.bodetech.com/technologies/touch-dna/touch-dna-overviewd, last visited June 30, 2010 (insisting that “Touch DNA is not Low Copy Number (LCN) DNA.”) (emphasis in original).

3. See, e.g., John Buckleton & Peter Gill, Low Copy Number. in Forensic DNA Evidence Interpretation 275 (John S. Buckleton et al. eds., 2005); Pamela J. Smith & Jack Ballantyne. Simplified Low-Copy-Number DNA Analysis by Post-PCR Purification, 52 J. Forensic Sci. 820 (2007).

4. But in the unreported case of United States v. Williams, No. CR 05-920-RSWL, 2009 WL 1704986 (C.D. Cal. June 17, 2009), a different prosecution expert “declared that, without any increase in the amplification cycles, the amount of input DNA is low enough to constitute LCN when the amount is less than .1 ng or .2 ng of input DNA.” Id. at *2.

5. 602 F. Supp.2d at 669 (“The one feature that all LCN processes have in common is the knowledge or expectation that less than the recommended amount of template DNA is being used.”).

6. Id. (“Using small amounts of template DNA (even without making any changes to the testing process itself) is all it takes for something to be in the LCN category.”).

7. Id. See also Williams, No. CR 05-920-RSWL (agonizing over definitions but concluding that the sample quantity was not low enough to constitute “LCN” under either party’s definition).