33A.1 Expert opinion evidence is admissible in criminal proceedings at common law if, in summary, (i) it is relevant to a matter in issue in the proceedings; (ii) it is needed to provide the court with information likely to be outside the court’s own knowledge and experience; and (iii) the witness is competent to give that opinion.
33A.2 Legislation relevant to the introduction and admissibility of such evidence includes section 30 of the Criminal Justice Act 1988, which provides that an expert report shall be admissible as evidence in criminal proceedings whether or not the person making it gives oral evidence, but that if he or she does not give oral evidence then the report is admissible only with the leave of the court; and Part 33 of the Criminal Procedure Rules, which in exercise of the powers conferred by section 81 of the Police and Criminal Evidence Act 1984 and section 20 of the Criminal Procedure and Investigations Act 1996 requires the service of expert evidence in advance of trial in the terms required by those rules.
33A.3 In the Law Commission report entitled ‘Expert Evidence in Criminal Proceedings in England and Wales’, report number 325, published in March, 2011, the Commission recommended a statutory test for the admissibility of expert evidence. However, in its response the government declined to legislate. The common law, therefore, remains the source of the criteria by reference to which the court must assess the admissibility and weight of such evidence; and rule 33.4 of the Criminal Procedure Rules lists those matters with which an expert’s report must deal, so that the court can conduct an adequate such assessment.
33A.4 In its judgment in R -v- Dlugosz & Others  EWCA Crim 2, the Court of Appeal observed (at paragraph 11): ‘It is essential to recall the principle which is applicable, namely in determining the issue of admissibility, the court must be satisfied that there is a sufficiently reliable scientific basis for the evidence to be admitted. If there is then the court leaves the opposing views to be tested before the jury.’ Nothing at common law precludes assessment by the court of the reliability of an expert opinion by reference to substantially similar factors to those the Law Commission recommended as conditions of admissibility, and courts are encouraged actively to enquire into such factors.
33A.5 Therefore factors which the court may take into account in determining the reliability of expert opinion, and especially of expert scientific opinion, include:
(a) the extent and quality of the data on which the expert’s opinion is based, and the validity of the methods by which they were obtained;
(b) if the expert’s opinion relies on an inference from any findings, whether the opinion properly explains how safe or unsafe the inference is (whether by reference to statistical significance or in other appropriate terms);
(c) if the expert’s opinion relies on the results of the use of any method (for instance, a test, measurement or survey), whether the opinion takes proper account of matters, such as the degree of precision or margin of uncertainty, affecting the accuracy or reliability of those results;
(d) the extent to which any material upon which the expert’s opinion is based has been reviewed by others with relevant expertise (for instance, in peerreviewed publications), and the views of those others on that material;
(e) the extent to which the expert’s opinion is based on material falling outside the expert’s own field of expertise;
(f) the completeness of the information which was available to the expert, and whether the expert took account of all relevant information in arriving at the opinion (including information as to the context of any facts to which the opinion relates);
(g) if there is a range of expert opinion on the matter in question, where in that range the expert’s own opinion lies and whether the expert’s preference has been properly explained; and
(h) whether the expert’s methods followed established practice in the field and, if they did not, whether the reason for the divergence has been properly explained.
33A.6 In addition, in considering reliability, and especially the reliability of expert scientific opinion, the court should be astute to identify potential flaws in such opinion which detract from its reliability, such as:
(a) being based on a hypothesis which has not been subjected to sufficient scrutiny (including, where appropriate, experimental or other testing), or which has failed to stand up to scrutiny;
(b) being based on an unjustifiable assumption;
(c) being based on flawed data;
(d) relying on an examination, technique, method or process which was not properly carried out or applied, or was not appropriate for use in the particular case; or
(e) relying on an inference or conclusion which has not been properly reached.
We were entirely unsurprised by the Government’s decision not to fund any of the proposed changes that arose from the Law Commission’s detailed and careful work investigating the reliability of expert evidence. Indeed, those who have observed the Ministry of Justice’s approach to limiting expert witness fees paid out of the legal aid fund – an unsophisticated and irrational scheme that pays no heed to the effect on access to justice – cannot be taken aback that the Government is unwilling to pay for the recommended reliability test. But the problems highlighted by the Law Commission remain, and it is the duty of all who are interested in justice to do what they can to ameliorate them.
Experts could, perhaps, do worse than to test their own opinions against the reliability factors set out by the Law Commission and embodied in this latest addition to the CrimPR.