The Grounds for Challenging Expert Evidence
Expert evidence in criminal proceedings is subject to challenge on four principal grounds. Each ground requires a different strategy and, in most cases, the instruction of a counter-expert to provide the evidential foundation for the challenge.
The witness does not possess the requisite expertise, or the subject matter does not require expert evidence. The court has an inherent power to exclude expert evidence that does not meet the threshold for admissibility.
The methodology underlying the opinion is not sufficiently reliable to be placed before the jury. The court applies a common law reliability test derived from R v Dlugosz [2013] EWCA Crim 2.
The report does not comply with the formal requirements of CrimPR Rule 19 — for example, it lacks the written declaration of the expert's duty to the court, or fails to identify the literature relied upon.
The expert has adopted the role of advocate rather than independent adviser. The Ikarian Reefer principles provide the framework for identifying and challenging advocacy bias.
The Reliability Test in English Criminal Courts
English criminal courts do not apply the US Daubert reliability test, but the Court of Appeal has developed a common law framework for assessing the reliability of expert evidence. The leading authority is R v Dlugosz [2013] EWCA Crim 2, where the court held that expert evidence is admissible only if there is a sufficiently reliable scientific basis for it.
The reliability challenge is most effective in cases involving novel or developing forensic disciplines — cell site analysis, facial mapping, gait analysis, and low copy number DNA are areas where reliability challenges have succeeded. A counter-expert who can identify methodological weaknesses and point to contrary scientific literature provides the evidential foundation for a successful challenge.
CrimPR Rule 19 Compliance Challenges
Criminal Procedure Rule 19 sets out the formal requirements for expert evidence in criminal proceedings. A report that does not comply with Rule 19 may be excluded or given reduced weight. The most common compliance failures are listed below.
| Rule 19 Requirement | Common Failure | Effect |
|---|---|---|
| Written declaration of duty to court (r.19.4(j)) | Declaration absent or not signed | Report may be excluded; expert may be required to re-serve |
| Statement of qualifications and experience (r.19.4(b)) | CV not appended; qualifications overstated | Weight reduced; admissibility challenged |
| Identification of literature relied upon (r.19.4(f)) | No bibliography; selective citation | Reliability challenge; counter-expert can identify omitted contrary literature |
| Statement of range of opinion (r.19.4(h)) | No acknowledgment of alternative views | Advocacy bias challenge; jury direction on limitations |
| Disclosure of instructions (r.19.4(a)) | Instructions not summarised | Potential for abuse of process argument where instructions were leading |
Advocacy Bias: Identifying and Challenging Partisan Experts
Advocacy bias — the tendency of an expert to adopt the position of the instructing party rather than provide independent advice to the court — is a recognised problem in expert evidence. The Ikarian Reefer principles, first articulated by Cresswell J in The Ikarian Reefer [1993] 2 Lloyd's Rep 68 and codified in CrimPR Rule 19, provide the framework for identifying and challenging biased expert evidence.
The Joint Expert Discussion as a Challenge Tool
A joint expert discussion under CrimPR Rule 19.6 is one of the most effective tools for challenging prosecution expert evidence. The discussion forces the prosecution expert to defend their methodology and conclusions in a structured setting, and any concessions made in the resulting joint statement can be deployed at trial.
The scope of the joint discussion should be agreed between the parties before the meeting. Frame the agenda around the specific points of disagreement identified in the respective reports. Avoid open-ended questions that allow the prosecution expert to re-state their position without engaging with the defence critique.
The defence expert must be fully briefed on the prosecution case theory, the prosecution expert's methodology, and the specific points the defence wishes to challenge. The expert should be provided with all relevant scientific literature and any additional material disclosed after the prosecution report was served.
The joint statement is a binding document that will be placed before the jury. Review it carefully before it is finalised. Any concession made by the prosecution expert — even a partial one — should be clearly recorded. Any area of disagreement should be precisely articulated so that the jury understands what the dispute is about.
The joint statement can be used in cross-examination to hold the prosecution expert to concessions made in the joint discussion. It can also be used to demonstrate to the jury that the prosecution expert's position is not universally accepted in the relevant scientific community.
Instructing a Counter-Expert
A counter-expert provides the evidential foundation for a challenge to prosecution expert evidence. The counter-expert reviews the prosecution report, identifies methodological weaknesses and compliance failures, and provides an independent opinion on the same subject matter. The letter of instruction to the counter-expert should be drafted carefully to ensure the expert addresses the specific points the defence wishes to challenge.
Frequently Asked Questions
On what grounds can expert evidence be challenged in criminal proceedings?
Expert evidence in criminal proceedings can be challenged on grounds of admissibility (the witness does not possess the requisite expertise), reliability (the methodology is not sufficiently reliable), compliance (the report does not comply with CrimPR Rule 19), or weight (the opinion is not adequately supported by the underlying data). The defence may also challenge the expert's impartiality where there is evidence of advocacy bias.
What is the reliability test for expert evidence in English criminal courts?
The Criminal Procedure Rules do not contain an explicit reliability test equivalent to the US Daubert standard, but the courts have developed a common law reliability framework. In R v Dlugosz [2013] EWCA Crim 2, the Court of Appeal held that the court must be satisfied that there is a sufficiently reliable scientific basis for the evidence to be admitted. Factors include whether the methodology has been tested, whether it has been subjected to peer review, the known or potential error rate, and whether it is generally accepted in the relevant scientific community.
What is a joint expert discussion and how is it used to challenge expert evidence?
A joint expert discussion under CrimPR Rule 19.6 is a meeting between the prosecution and defence experts to identify areas of agreement and disagreement. The discussion produces a joint statement that is served on the court. The joint discussion is a powerful tool for challenging prosecution expert evidence because it forces the prosecution expert to defend their methodology and conclusions in a structured setting, and any concessions made in the joint statement can be used at trial.
Can the defence instruct its own expert to challenge prosecution expert evidence?
Yes. The defence has the right to instruct its own expert to review the prosecution expert's report and provide a counter-opinion. The defence expert must comply with the same duties as the prosecution expert under CrimPR Rule 19 — the overriding duty is to the court, not to the instructing party. LAA prior authority is available for defence expert fees in legally aided cases.
What is advocacy bias in expert evidence and how is it identified?
Advocacy bias occurs when an expert adopts the role of advocate for the instructing party rather than independent adviser to the court. Indicators include selective presentation of evidence, failure to address contrary literature, use of language that goes beyond the data, and a pattern of consistently supporting the instructing party across multiple cases. The Ikarian Reefer principles and the duties set out in CrimPR Rule 19 provide the framework for identifying and challenging advocacy bias.