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Introduction to the law on expert evidence

Chris Maxwell, Stewart Bayles, expert evidence, opinion evidence
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Essentially, the common law prohibits the giving of opinion evidence, that is, the stating of an inference that is to be drawn from what has been observed or reported as fact. And the assumption is that witnesses should give evidence only as to what they observed. That is the facts and the prohibition of opinion evidence assumes that we can draw a sharp distinction between inferences and the facts on which they’re based. The drawing inferences is said to be for the judge or the jury based on what the witness says, which should be purely factual. So experts, when they give evidence, are prevented from giving evidence about matters which are within the jury’s own knowledge.
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But there is this critical exception for expert evidence on matters calling for special knowledge or skill. And we accept that judges and jurors are not necessarily equipped to draw inferences which require that degree of knowledge. So provided an expert witness has the specialist knowledge, they can provide an opinion on facts.
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Why is expert evidence important in criminal trials? We’re talking here about evidence of a range of kinds, including scientific, medical, social science, financial. Of particular concern is evidence of a scientific, forensic and medical character which is routinely led in criminal trials. And why is it important? Well, because evidence of that kind can assist the jurors who have no scientific training to determine a range of questions which will bear on criminal liability. For example, scientific evidence of that kind can assist the jury to understand what the evidence signifies in a criminal trial.
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Forensic evidence might tell you something about the sequence of events, the likelihood of a particular person having been in touch with that person or that place, and whether there’s a causal link between two events. The essence of the admissibility of expert opinion at common law is that it furnishes the court with scientific information likely to be outside the jury’s experience. Put another way, it provides the trier of fact, whether it’s a judge or a jury with an inference from the facts which they wouldn’t themselves be able to formulate. Now, of course, if expert evidence is to prove anything satisfactorily, it must be robust and well founded.
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And that’s crucial to the fairness of the criminal justice system, as well as maintaining public confidence.
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I think the first proposition to understand is that expert evidence is a form of opinion evidence and in my view should not be treated as fact or conclusion. Expert evidence will always be opinion evidence and it should be treated as opinion evidence. Opinion evidence is not ordinarily admissible in court. Expert evidence is an exception to the opinion rule. That exception is because there is an area of expertise, because the witness has expertise in that area and because the opinion being expressed is within their area of expertise. But you have the right to challenge each of those matters.
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So maybe a golden rule in terms of your approach to an expert witness is just because the expert says that something is so, that doesn’t necessarily make it so. The expert witness doesn’t just walk into the courtroom with absolute authority on a subject. The expert should still be called upon to demonstrate why they hold this opinion, how they got to it, what is it based on, to show the workings, to show the basis for the opinion.
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In my experience, most legitimate expert witnesses who have a robust and rigorous approach to their area of expertise and the giving of evidence are very happy to talk about the limits of their area of expertise, the limits of the information they were given, how their opinion is based on the information they were given and limited to that information. They are very happy not to be drawn outside these areas. Most expert witnesses are very comfortable talking about matters within their area of expertise and less comfortable talking about matters that are outside their area of expertise.
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And they will and should readily concede when something is outside their area of expertise and they should concede that their opinion is confined only to the information that they have been provided with. Often, I find that a witness who confidently offers opinions or makes bold statements on a broad, sweeping range of issues is usually a sign of someone who is not a rigorous and legitimate expert witness.

Justice Chris Maxwell and Judge Stewart Bayles begin by providing an introduction to the law on expert evidence, its origins as an exception to opinion evidence, and its role in trial.

What are the different types of expert evidence you have seen in civil and criminal cases? List them in the comments section.

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