What Is An Expert Witness

Definition of Expert Witness

Expert evidence is admissible to furnish the court with information which is likely to be outside the experience and the knowledge of a judge or jury (Criminal Practice Directions 2023 7.1.1 Expert Evidence).

An expert witness can provide the court with a statement of opinion on any admissible matter calling for expertise by the witness if they are qualified to give such an opinion.

The Duty of an Expert Witness

The duty of an expert witness is to help the court to achieve the overriding objective by giving opinion which is objective and unbiased, in relation to matters within their expertise. This is a duty that is owed to the court and overrides any obligation to the party from whom the expert is receiving instructions - see Criminal Procedure Rules 2020 Part 19. (CrimPR 19.

CrimPR 19.2(3)(d) also obliges all experts to disclose to the party instructing them anything (of which the expert is aware) that might reasonably be thought capable of undermining the expert’s opinion or detracting from their credibility or impartiality.

The Criminal Practice Directions 2023 provides examples of information at 7.1.4 that should be disclosed by all experts under CrimPR 19.2(3)(d) including:

(a) any fee arrangement under which the amount or payment of the expert’s fees is in any way dependent on the outcome of the case;

(b) any conflict of interest of any kind, other than a potential conflict disclosed in the expert’s report;

(c) adverse judicial comment;

(d) any case in which an appeal has been allowed by reason of a deficiency in the expert’s evidence;

(e) any adverse finding, disciplinary proceedings or other criticism by a professional, regulatory or registration body or authority, including the Forensic Science Regulator;

(f) any such adverse finding or disciplinary proceedings against, or other such criticism of, others associated with the corporation or other body with which the expert works which calls into question the quality of that corporation's or body's work generally;

(g) conviction of a criminal offence in circumstances that suggest:

a lack of respect for, or understanding of, the interests of the criminal justice system (for example, perjury; acts perverting or tending to pervert the course of public justice),

dishonesty (for example, theft or fraud), or

a lack of personal integrity (for example, corruption or a sexual offence)

(h) lack of an accreditation or other commitment to prescribed standards where that might be expected;

(i) a history of failure or poor performance in quality or proficiency assessments;

(j) a history of lax or inadequate scientific methods;

(k) a history of failure to observe recognised standards in the expert’s area of expertise;

(l) a history of failure to adhere to the standards expected of an expert witness in the criminal justice system.

CrimPR Rule 1 states that the 'overriding objective' of the rules is for criminal cases to be dealt with justly. This rule places certain obligations on each participant in the criminal justice process, which includes expert witnesses. These obligations require the participant to deal with the case justly and in an efficient and expeditious manner, complying with the Rules and any other direction that the court makes, and advising the court should they or any another participant not comply with such rules and directions.

Admissibility of Expert Evidence

The general rule is that witnesses should only testify in relation to matters within their knowledge. Evidence of opinion or belief is inadmissible. However, exceptions have been made by statute and at common law in relation to expert evidence.

Statute

Section 30 of the Criminal Justice Act 1988 states that an expert's report is admissible as evidence of fact and opinion, whether or not the expert attends court to give oral evidence. If it is not proposed to call the expert witness, the leave of the court must be obtained prior to introducing it.

In considering whether to grant leave, the court will have regard to:

The contents of the report;

The reasons why it is proposed that the expert will not give live evidence;

The risk that it may not be possible to controvert statements in the report if the expert does not attend;

Any unfairness to the accused; and

Any other relevant circumstances - which, in practice, should include consideration as to whether the Criminal Procedure Rules have been complied with and the extent to which the evidence would have been admissible at common law.

Common law

Expert opinion evidence is admissible at common law where:

It will be of assistance to the court

For expert opinion to be admissible it must be able to provide the court with information which is likely to be outside a judge’s or a jury's knowledge and experience, but it must also be evidence which gives the court the help it needs in forming its conclusions.

The role of the expert is to give their opinion based on their analysis of the available evidence. The Bench or jury is not bound by that opinion but can take it into consideration in determining the facts in issue.

If the expert is seeking to advance an opinion which is not relevant to an issue in the case or which might be deemed a matter of common sense upon which the jury could reach its own conclusions, then the opinion of an expert will be inadmissible.

For instance, in R v Turner (1975) 60 Cr App R 80, the issue as to the credibility of a witness was a matter for the jury. Psychiatric evidence as to how an ordinary person who was not suffering from a mental disorder would react to a given situation was held to be inadmissible.

The expert has relevant expertise

The individual claiming expertise must have acquired by study or experience sufficient knowledge of the relevant field to render their opinion of value.

The court is concerned that evidence should not be given by experts who are, patently unqualified or little more than ‘enthusiastic amateurs. More commonly, it is vital to ensure that an expert does not give evidence in relation to matters outside of their expertise - see R v Clarke & Morabir [2013] EWCA Crim 162, a case where an expert in fractures and bone disease gave an opinion as to cause of death, in circumstances where the Court of Appeal held that he "did not have the experience or expertise to consider all of the causes of death" in the way that a Home Office registered forensic pathologist would.

However, where the witness possesses relevant formal qualifications in the field of study, challenges to admissibility on the basis of lack of expertise will rarely succeed. Challenges may be more frequent if the expert has gained knowledge based upon experience or informal studies, but, even here, that knowledge can be of assistance to the court.

In R v Hodges [2003] EWCA Crim 290, the evidence of a police officer with years of experience in the investigation of drugs offences and using knowledge acquired from informants and arrested suspects was admissible in relation to the issue of the normal manner of supply of heroin, the usual price and the quantity of drugs that would constitute a supply for personal use. In R (Doughty) v Ely Magistrates' Court [2008] EWHC 522, the fact that a defence expert had not recently handled a speed detection device of the type used in the case before the Magistrates, nor had he attended the same approved courses as the prosecution expert was a matter which went to the weight to be attached to his evidence and was not a reason for ruling his evidence to be inadmissible.

If there is evidence that a witness has been discredited, then the court may need to examine this undermining material in order to form a view as to whether the witness can still be relied upon, or whether he is so discredited that his evidence should be ruled inadmissible. Alternatively, this material can be placed before a jury to allow them to assess weight to be attached to the evidence, as opposed to its admissibility.

The expert is impartial

The expert must be able to provide impartial, unbiased, objective evidence on the matters within their field of expertise. This is reinforced by Rule 19.2 of the Criminal Procedure Rules which provides that an expert has an overriding duty to give opinion evidence which is objective and unbiased.

An expert is independent of the parties to the proceedings and should not be seen to usurp the role of the advocate in the proceedings by seeking to make submissions to the court - see, for example, the case R v Cleobury [2012] EWCA Crim 17, where a DNA expert sought in his report, prepared for the purposes of an appeal, to criticise the judge's summing-up in the original trial and commented on the importance of the forensic evidence to the case as a whole.

A potential conflict of interest does not operate so as to automatically disqualify a witness from giving evidence. The key question is whether the evidence that the witness gives is impartial and not, for example, whether he works for the same company as the defence expert. However, it is vital that any potential conflict is disclosed to the court and other parties to the proceedings by the party wishing to call the expert as soon as possible, so that an informed decision can be made as to whether the expert is impartial and what weight to be attached to his evidence - see Toth v Jarman [2006] EWCA Civ 1028 and R v Stubbs [2006] EWCA Crim 2312. In Stubbs, a bank employee was charged with conspiracy to defraud his employer using its online banking facility. The Court held that expert evidence as to the operation of that system could be given by another employee of the bank. It was admissible as long as the witness’s status was explained to the jury so that they could take this into account in assessing the weight to be attached to the evidence.

The expert's evidence is reliable

There should be a sufficiently reliable scientific basis for the expert evidence, or it must be part of a body of knowledge or experience which is sufficiently organised or recognised to be accepted as a reliable body of knowledge or experience.

The reliability of the opinion evidence will also take into account the methods used in reaching that opinion, such as validated laboratory techniques and technologies, and whether those processes are recognised as providing a sufficient scientific basis upon which the expert's conclusions can be reached. The expert must provide the court with the necessary scientific criteria against which to judge their conclusions.

In satisfying itself that there is a sufficiently reliable basis for expert evidence to be admitted, the court will be expected to have regard to Criminal Practice Directions 2023 at 7.1.2which states:

"7.1.2Factors which the court may take into account in determining the reliability of expert opinion, and especially of expert scientific opinion, include:

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;

the validity of the methodology employed by the expert;

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);

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;

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 peer- reviewed publications), and the views of those others on that material;

the extent to which the expert's opinion is based on material falling outside the expert's own field of expertise;

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);

if there is a range of expert opinion on the matter in question, where in the range the expert's own opinion lies and whether the expert's preference has been properly explained; and

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.

"7.1.3: In addition, in considering reliability, and especially the reliability of expert scientific opinion, the court must be astute to identify potential flaws in such opinion which detract from its reliability, such as:

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;

being based on an unjustifiable assumption;

being based on flawed data;

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

relying on an inference or conclusion which has not been properly reached."

If satisfied, having regard to the Criminal Practice Direction, that the evidence is sufficiently reliable, the court will leave the opposing views to be tested by the jury. The court will be keen to ensure that the jury is not deprived of useful relevant evidence that will assist them in determining the issues in the case. In these circumstances, the court may wish to allow the evidence to be admitted in circumstances where the jury is provided with the underlying information to allow them to judge the weight to be attached to it.