Three cases have re-considered position of the expert evidence and the importance or otherwise of experts expressing the strength of their opinion on a statistical or numerical basis. Although these cases concerned DNA evidence, the reasoning could be applied in relation to any evaluative expert opinion where a statistical analysis is not possible, and as such has potentially wider implications.
The Court of Appeal heard the appeals of Duglosz, Pickering and MDS together as each case, although these cases had quite different factual circumstances, the Court took issue with the admissibility of evidence in relation to Low Template DNA samples from mixed profiles. The particular issue was whether expert evidence was admissible when it was impossible for the expert to express the strength of the evidence in statistical terms, and could only give an evaluative opinion.
In each case the Crown relied on evidence from analysis of Low Template DNA from a mixed sample. Low Template DNA is the term given to very small samples of DNA which require additional amplification during analysis in order to produce a full DNA profile. Mixed samples refer to samples of DNA where there are at least two contributors.
The availability of only mixed samples meant that statistical or numerical analysis of random matching was not possible in any of the cases. No expert was able to present their analysis in a format which set out the chances of the defendant’s DNA being present by chance.
Applications to exclude the expert evidence were made by the defence lawyers in each of the cases on the basis that the jury could not properly assess the relative significance of the presence of the defendant’s DNA without expert evidence in statistical format which set out the probability of a match happening by chance.
In the case of Duglosz the Crown’s expert said that it would be “rare” to find all 20 components of the defendant’s DNA in a mixed profile if he had not been involved in the offence. The defence expert stated it would be “somewhat unusual”.
In Pickering the Crown’s expert stated that the findings were more likely to be consistent with the complainant’s account than with Pickering’s account, but that he could not express a view on the strength of the support. No independent forensic evidence was relied upon by the defence.
In MDS the Crown’s forensic expert expressed her findings according to a hierarchy of views, stating “in my opinion if MDS had contributed a portion of DNA then…the result is what I might expect” and “If MDS had not contributed any of the DNA to the sample, then I consider it unlikely that all of his components would be represented...by chance alone.”
The Court of Appeal considered the format of the forensic evidence, whether there was a need for statistical analysis and the status of the expert in each case. The Court concluded that “evaluative evidence is admissible provided that the judge is satisfied that the expert giving that evidence has a proper basis for giving the evaluative evidence based upon his or her experience”.
This means that where the nature of the evidence does not permit statistical analysis to be undertaken, the status of the expert is of paramount importance. It would be wise for defence lawyers to consider carefully the qualifications and experience of any potential expert witness before instruction, and to have particular regard to the number of times the expert has undertaken the specific analysis required. These factors will form the basis of any assessment of the relative weight of prosecution and defence expert evidence, and will be central to arguments concerning its admissibility. In some cases, especially those involving serious sexual or violent offences, this could mean the difference between conviction and acquittal