Recent updates to the Criminal Procedure Rules and Practice Directions encourage the court to be astute in identifying potential flaws in expert opinion (CrimPD 19A.6). This seems a reasonable suggestion, but may be rather more difficult to implement. What do Counsel need to know in order to determine the admissibility of DNA findings? CrimPD 19A.5 suggests considering a variety of factors in determining the reliability of expert opinion.
These are complex issues relating to matters including validity of methods, reliance on inference, degree of accuracy and precision involved in formulating opinions, and indeed, the degree of uncertainty. Counsel are likely to require expert assistance with such considerations. Consultant forensic scientist and chartered forensic practitioner, Sue Carney, provides here, her Top 5 tips to assist Counsel in understanding and challenging DNA expert reports.
1. Check the status of the expert report
DNA findings are frequently presented these days in an abbreviated format: the Streamlined Forensic Report (SFR). The initial, and most basic of these, the SFR1 (aka the MG22b), merely reports a DNA match. No context is provided for the match; usually only an exhibit reference and the name of the matching individual. SFR1s are automatically generated. Indeed, the named ‘author’ of these reports is often the seizer of the exhibit, rather than a DNA expert.
SFR1s are not evidential. Should the prosecution intend to rely on DNA findings then they are advised to request their expert produce a full evaluative statement. In practice, defence are frequently provided only with an SFR1 at the crucial point at which they must decide whether or not to instruct their own DNA expert. Expert intervention is encouraged at this stage and most defence-instructed experts will provide a free quotation and initial guidance.
The SFR2 (or MG22a) is an abbreviated report intended for evidential use. The degree of abbreviation may vary depending on which forensic service provider is the issuer. Whilst some SFR2s include an interpretation of the findings, this is often limited to the most basic ‘source level’ question: From whom has the biological material originated? Frequently, the more complex issues of how and when the material was deposited are not addressed.
2. Understand the statistic
A DNA match is often evaluated numerically. This value can be presented in one of two closely connected formats: the match probability or the likelihood ratio. For a full DNA profile match, the match probability is estimated to be in the order of one in a billion (where a billion is a thousand million or 109). This is the chance of obtaining the DNA profile if the sample has originated from a person other than and unrelated to the matching person. In other words, if the DNA is not from the matching person and the match is by chance. The equivalent wording in likelihood ratio terms would be to describe the DNA match as a billion times more likely if the DNA were from the matching person rather than another unrelated person.
A billion is an estimation. A full profile match is, in reality, many orders of magnitude rarer. A billion is considered to be a fair and reasonable reflection of the value of a DNA match and has been used as a maximum cut-off value to express this strength of evidence since the previous DNA profiling system was in routine use. A billion continues to be applied to current systems, which are considerably more discriminating than previous models.
Counsel are advised to be cautious when elucidating the relevance of a DNA statistic. The value of a billion cannot be used to consider the number of potentially matching individuals within a population of a certain size. A billion does not relate to a population in this way. Instead, it is a value derived from the comparison of two probabilities. Further, the statistic relates solely to the likelihood of the DNA match if the DNA were from a particular person. It does not equate to the probability that the DNA is from that person. Such erroneous thinking is termed “the transposed conditional”, or more commonly, “the prosecutor’s fallacy”.
3. Is it a mixture?
Increased sensitivity of DNA profiling in recent years has led to the detection of far more mixtures of DNA. The type and complexity of a DNA mixture will determine whether any statistical evaluation can be applied, although software solutions are now available that statistically resolve complex DNA mixtures of up to 4 or even 5 contributors, that were hitherto unable to be evaluated.
If a mixture of DNA is particularly low-level, it may not be possible to accurately determine the number of contributors, in which case statistical evaluation may be ill-advised.
As the complexity of a DNA mixture, i.e. the number of contributors increases, then so too does the chance that any and every reference DNA profile will share a significant number of common components. In short, as complexity of a mixture increases, potential evidential usefulness decreases.
4. Attribution?
Can DNA detected in a particular circumstance be attributed to a particular body fluid or tissue type? This may depend on both the circumstances of the case and the results of tests, which may be limited by their sensitivity and / or specificity. Some test results might indicate, rather than confirm, the presence of a substance, and any attribution of DNA to that substance must take account of some degree of uncertainty. Indeed, forensic science rarely deals in absolutes.
Generally, if a mixture of DNA is detected in conjunction with a detectable body fluid, then it is not usually possible to attribute the DNA of any particular contributor to the body fluid. There are a few exceptions, such as the presence of a male-female mixture alongside the detection of semen.
If a DNA profile is considered to be low level, nearing the limits of detection, then it may not be possible to attribute it to a body fluid, nor to determine how or when it was deposited. Given the ubiquitous nature of DNA; its expected presence on surfaces with a high degree of human contact, then it is possible that low level DNA detected within the context of a criminal investigation may not relate to, or have been deposited during the incident at all.
5. Transfer issues
Mechanisms of DNA transfer from contact are not yet fully understood. A body of research dating back more than 20 years indicates that DNA transfer from the hands involves multiple factors including the conditions of transfer (duration, force, friction, moisture and the like), the cleanliness of the hands and the activity of the individual prior to transfer. Research suggests that DNA comes to be present on the fingers via a variety of potential mechanisms.
The phenomenon of indirect transfer of DNA, where an individual’s biological material is deposited onto a surface indirectly, via some other surface or object, has been widely reported in the literature. The donor of indirectly transferred DNA has not had any contact with the object. Expert reports may refer to directly and indirectly transferred DNA as if, somehow, they are distinguishable. However, generally, it is not possible to determine from a DNA profile, whether DNA has been deposited directly or indirectly.
The surfaces of any object with an habitual user may be expected to bear a build up of their DNA over time. DNA can also be removed from an object by subsequent use and a change of user will inevitably lead to a gradual replacement of the previous user’s DNA. Conversely, an individual may handle an item without depositing their DNA in sufficient quantity to detect, yet at the same time, may deposit a substantially greater amount of someone else’s DNA.
Given these considerations, a DNA match may not be in question, rather, the circumstances surrounding the incident and any version of events upon which the accused intends to rely become crucial in addressing the issue of how the matching DNA came to be present.
Sue Carney, MPhil, BSc (Hons), MCSFS, ChFP (Biology) has been a forensic scientist for 20 years; 10 years employed by the former Forensic Science Service, and 10 years as an independent consultant forensic scientist and proprietor of Ethos Forensics. Sue is widely instructed by the defence in England and Wales, and on occasion, in international jurisdictions. She deals with matters involving DNA profiling and body fluids findings, including blood pattern interpretation, and has given expert testimony at crown court in many cases.
Sue can be contacted for a no obligation quotation and initial consultation at sue.carney@ethosforensics.com
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