Understanding relevance, admissibility and weight of evidence

The question at the threshold is, is the evidence relevant to a fact in issue in the trial? If it is, it’s admissible. The question of what weight that evidence will be given is a matter for the fact finder. And at the threshold, asking the admissibility question, it’s not for the judge who decides admissibility to speculate about ways in which the evidence might be used. Rather, the judge must assess relevance by taking the evidence at the highest level at which it can reasonably be put by the party seeking to rely on the evidence. It’s not for the judge to make an assessment of probative value at the stage of deciding whether it’s relevant.

Even the possibility of fabrication evidence doesn’t affect relevance at the threshold. And when it’s said that judges in jury trials in determining admissibility have regard to the weight of the evidence, they are having to consider it on the assumption that the evidence is believed, what weight would it reasonably be given if the jury accepted it? And under our uniform evidence legislation, we have the same distinction between admissibility on the grounds of relevance and weight, which goes to the extent to which it will prove something. So relevance depends on whether the evidence, if accepted, would increase the probability of the existence of a fact in issue. Is the evidence capable of doing that? Could it rationally have that effect?

The judge, as I said, doesn’t assess how much weight it should be given, it assumes that it will be given full weight. That’s the test at the threshold for admissibility. Under our legislation as at common law, there are, of course, grounds for exclusion of relevant evidence. Most obviously on the grounds of unfairness. Would the evidence be likely to create a risk of unfair prejudice, which would outweigh its probative value?

Opinion evidence and of course, expert evidence is that branch of opinion evidence we’re talking about, is like all other evidence subject to the principle of relevance. So at the threshold, the judge has to accept that the experts evidence would affect directly or indirectly the existence of a fact in issue in the trial. For example, was the accused at that place at the time. But at common law, there are a number of additional requirements for the admissibility of expert evidence. And I’ll run through those briefly. In addition to the relevance of the expert opinion to a fact in issue, there must be evidence capable of proving the facts on which the expert based her opinion.

The witness must have disclosed the facts on which the evidence was based. And crucially, in this area, the expert has to demonstrate that she has expertise in a relevant field of expertise, sufficiently organized or recognized to be accepted as a reliable body of knowledge or experience. As I said earlier, to be admissible, the opinion can’t relate to a matter of common knowledge. It must relate to that matter of specialized knowledge.

Now, first of all, I want to start thinking in terms of two ways in which you might challenge expert evidence before the court. The first, why is that you might go about challenging the admissibility of that evidence completely. So that if successful, the evidence is not admitted in the trial at all. And the second way that you might go about challenging the expert evidence is that, if the evidence is admitted into the trial, you might then challenge that evidence in the running of the trial through cross-examination of the witness, with the intention of limiting the impact that, that evidence will have in the case against your client.

And I think the second category can even be broken down into a further two categories. And first of all, if you mount a complete attack on the evidence where if you are successful in doing that, you might get the evidence to the point where the court might give the evidence no weight or very little weight, after you’ve attacked and undermined the evidence. Or secondly, your goal in cross-examination could be to try and shape or influence what the evidence is so that the evidence itself might still be before the court, but it might be less bad for your client’s case. It might be neutralized, or in some cases, it might even be able to help your client’s case.

So if you start thinking about these things early on, it might take some time in the preparation of a case before you come to form a view about this. But ultimately, you have to work out, what is your goal with the expert evidence? Is your goal to have the evidence excluded as inadmissible? Is your goal to attack and to undermine the evidence in the trial? Or is your goal to try and shape and influence what the evidence is in the trial? Or it could be a combination of all three of these.