Challenging expert evidence in court

In this video, I want to do two things. First, I want to talk a little bit about when you would engage your own expert witness and how to use your own expert witness in a trial. And secondly, I want to have a look a bit closer at some specific approaches that you might take in court to challenge expert evidence. So your own expert witness, there’s at least two reasons why you would engage your own expert witness in the lead up to a criminal trial. The first reason is your expert can assist you in understanding the prosecution expert report. Your expert can help you analyse, critically analyse the prosecution expert report.

They can tell you what kind of information you should request from the prosecution expert, and they can assist you in preparing your cross-examination of the prosecution expert witness. They can point out weaknesses and areas that you might cross-examine on, and they can provide you with propositions that you can put to the prosecution witness in cross-examination. The second reason, of course, is that you might call your own expert witness to give evidence in the trial. Now, let’s just pause and think about this for a moment.

If you have a proposition that you want to put to the prosecution expert witness and you put that proposition and they agree with it, then you don’t need to call your own expert witness to give that evidence. That is now already in evidence. However, if you put some propositions to the prosecution expert and they don’t agree with them if they maintain their position and if that position is different from what you want in the evidence, then that evidence stands and you have to then expect that the court will accept that evidence as it stands. So if you want to dispute that evidence, then you have to call your own expert witness to give that evidence.

Now, the second thing that I want to talk about in this video is to have a closer look at some of the ways that you might challenge an expert witness in court. The first thing on this subject is whatever the area of expertise, whatever the opinion is, your ability to challenge that evidence and that opinion will always be highly dependent on your knowledge of the area of expertise, your knowledge of the subject matter. You will need to have an understanding of the subject matter in dispute. Now, this is not always going to be an easy thing to do. This might take a lot of work in preparation .

Reading of the prosecution report, researching the area of expertise, doing online research, reading articles, often just simply sitting before the computer and googling terms can help a lot. You can look to see yourself whether the opinion is properly justified and whether it’s properly based on what they say it’s based on. I always find the question what is that based on is a good way to keep digging and to test an opinion. The witness makes an assertion in evidence. Ask them what is that based on. Get them to justify and explain their opinion. Sometimes a witness will say, that’s based on my twenty five years of experience in this field.

Now, in my experience, that answer usually means that it’s not based on anything in particular. It’s really more of a form of speculation or guesswork. An expert needs to do better than that. They need to be able to explain and to justify and to demonstrate how they got to their opinion. Try to pin the expert witness down to what are the precise facts that their opinion is based on. Because if through other evidence you can undermine some of these facts, then you might render their opinion irrelevant because their opinion is based on facts that are not proven. If there are parts of the statement that you don’t understand, ask them, ask the witness, what does this mean.

Get them to take you through the statement, explain the terms. Explain the process that they engaged in to arrive at their opinion. Sometimes an expert witness will make assumptions. There might be a gap in their knowledge of the facts. There might be a gap in the information they have been provided with, and so they make an assumption to fill in that kind of gap. If so, are their assumptions reasonable? Are they consistent with the evidence or is there evidence in the trial that suggests that the assumptions are wrong or should not have been made?

If experts are not used to being challenged in this way, then you have to start doing it and you have to start getting them used to being challenged.