In the Media

Complex DNA mixtures as evidence

PUBLISHED August 8, 2013

Monday 12 August 2013 by Ian Evett and Sue Pope

The judgment in R v Dlugosz ([2013] EWCA Crim 2) concerning DNA profiles from mixed samples proposes that if the scientist cannot provide a numerical assessment of the weight of the evidence, the jury should form their own assessment based on the number of matching components, assisted by a qualitative opinion based on the experience of the scientist.

When a crime sample consists of good-quality DNA from a single person, its comparison with a single defendant is straightforward. If the two profiles are the same, an evidential weight is assigned by addressing two propositions:

  • prosecution - DNA in the crime sample came from the defendant; or
  • defence - DNA in the crime sample came from some unknown person.

The scientist considers two questions. What is the probability I would have made those observations if the prosecution proposition were true? What is the probability I would have made those observations if the defence proposition were true? The ratio of the answers is the 'likelihood ratio', a measure of the weight of evidence in favour of the prosecution proposition. Here the answer to the first question is assigned a value of one - a full match between the crime and defendant's profile is expected.

The second is the probability that an unknown person would have the same profile as the crime profile - the 'random match probability' (RMP). The weight of evidence in favour of the prosecution is the simple inverse of the RMP - the smaller the RMP, the greater the weight of evidence.

With a mixture, the propositions are inevitably more complicated. An example might be:

  • Prosecution - the defendant is one of three contributors to the mixture; or
  • Defence - the mixture is from three unknown persons.

The answer to the first question - what is the probability of the observations if the prosecution proposition were true? - cannot be one, and the answer to the second is not provided by the RMP. Sometimes both questions can be addressed numerically and a likelihood ratio calculated. However this is not always possible.

Counting components

The number of 'alleles' (in simple terms, an alternative form of genes) in a mixed profile that are also in the defendant's profile can be counted, and this has been suggested as a basis for a qualitative assessment of evidential weight (for example, R v Walsh ([2011] NICC 32).

If this method offered a simple means of providing a balanced assessment of weight of evidence then forensic scientists would already be using it. However, whereas the number of matching components may provide part of the means for addressing the second question, it provides no assistance in addressing the first question. As in R v Clark ([2000] Crim 54) and R v George ([2007] EWCA Crim 2722), although the probability of the observations may be small if the defence proposition be true, it will also be small if the prosecution proposition be true - failure to clarify this for the jury is prejudicial.

Qualitative opinions

The ruling assumes that a scientist who has quantitatively evaluated a large number of mixtures cases will have the knowledge to assign a reliable qualitative opinion of weight of evidence in a case that is too complex for a quantitative assessment. However, there is no scientific basis for this belief - no scientific literature provides a reliable methodology, scientists are not trained to make such assessments and there is no body of standards to support them. Casework experience is not a substitute. The true composition of the DNA result in any given case cannot be known so it does not provide a reliable control for learning purposes.

When this issue arose in other areas of forensic science, for example handwriting comparison, the solution was to measure competency by means of programmes of assessment of known samples under controlled conditions. In our opinion, the way of assessing whether or not qualitative opinions in DNA mixtures provided reliable and robust evidence would be by means of a national programme of assessing scientists' interpretation of specially constructed DNA mixtures under controlled conditions against agreed standards.

In summary, in the case where the scientist is unable to provide a numerical assessment of the weight of evidence relating to a DNA mixture comparison:

  • Presenting the number of matching components between a defendant and a DNA mixture fosters a prejudicial view of the evidence.
  • Scientists are not trained to provide qualitative assessments of evidential weight of DNA mixtures, and no system exists for assessing the robustness of such assessments.

Ian Evett is an expert in the interpretation of scientific evidence and Sue Pope is a DNA expert at Principal Forensic Services