Knowing the rules of expert evidence can protect yourself from attack!

Lawyers who know the rules of expert evidence well, wield these rules like a weapon.  The rules can be a very powerful weapon in the armoury of a litigator.   When you become an expert witness, this weapon can be directed at your evidence to try to undermine your opinion or to convince a court that they should not hear your evidence at all.  

I wanted to give you an overview of what lawyers think about when trying to attack an expert’s evidence.

Firstly, by way of background, witnesses should not give evidence of their opinion.   The exception to this rule is an “expert”.  They can provide an opinion if it assists the court to understand technical or scientific issues.  But the courts, over the years, have worried that the expert could be seen to take over the decision-making role of the judge or the jury.   So, to prevent the expert from usurping the role of the judge or jury, rules of expert evidence have developed.  

Rules of Expert Evidence

There are many useful expositions of these rules in case law.  (e.g., Makita (Australia) Pty Ltd v Sprowles (2001) 52 NSWLR 705; Dasreef Pty Ltd v Hawchar (2011) 243 CLR 588; Idoport Pty Ltd v National Australian Bank Ltd [1999] NSWSC 828). 

The rules of expert evidence can be summarised as:

1. Expertise Rule: The Expert must have specialised knowledge based on their training, study, or experience.
2. Area of Expertise Rule: The knowledge must be within a recognised, organised, or accepted field of study.
3. Common Knowledge Rule: The expert must not give evidence of matters that are within the judge’s or jurors’ own experience or knowledge.
4. Basis Rule: The facts upon which the opinion is based must be proven to the satisfaction of the court.
5. Ultimate Issue Rule: Experts should not swear the issue.

Lawyers thoughtfully consider the rules of expert evidence before spending their client’s money on experts.   The opposing lawyer, who has a sound understanding of the rules of expert evidence, will also likely use these rules as a weapon from their arsenal.   They will use it to attack or undermine their opponent’s experts.  

Recent examples where expert opinion has been rejected

There are several recent cases where courts have refused to admit or accept opinions of experts because they have breached the rules of expert evidence.  

In Menz v Wagga Wagga Show Society Inc [2020] NSWCA 65, the Court of Appeal confirmed that the trial judge was correct to exclude the opinion of a competitive horse rider, trainer, and judge.   This witness gave an opinion that the show society ought to have had more marshals and better supervised children.   She asserted this would have prevented the children making loud noises that startled the horses which led to a rider being injured.    The Court of Appeal agreed that the witness did not demonstrate that she had specific expertise to comment on the management or staffing of agricultural shows.

In MacGowan v Klatt [2019] QSC 222, during a family provision application, the Queensland Supreme Court heard evidence from two legal experts on the status of Vanuatu law regarding adoption.  One of the experts gave her opinion in a manner that breached the ultimate issue rule.  The court preferred the other expert. 

In Hawkesbury Sports Council v Martin [2019] NSWCA 76, the Court of Appeal determined that the primary judge erred when admitting the opinion of an engineer and building consultant.   The witness gave opinion on whether a pedestrian ought to have perceived a steel cable strung between bollards around a sporting field.  The Court of Appeal determined that the witness’s opinion (on human visual perception) was outside his expertise and was a matter of common knowledge.

In Brown v Daniels [2018] QSC 209, the Court refused to admit the opinion of an engineer about who caused a motor vehicle accident.   The Court held that the opinions were not based on science.   They were “simply conclusions which are drawn from the physical evidence”.   The court was in as good a position as the engineer to consider the evidence and draw conclusions about the movement of the vehicles at the time of the accident.  

Make thoughtful and strategic decisions when deciding to become an expert witness

An expert witness who understands the rules of expert evidence, and how they can be wielded by lawyers, will stand a better chance against a flank attack.  

If the expert understands the scope of their role, where their expertise starts and ends, and where the boundaries of their opinion evidence should be, then they will be more immune to attack. 

The expert who does not understand the scope of their role and their function in the legal system makes themselves unnecessarily exposed to criticism and vulnerable to attack.   

The best preparation for giving expert evidence is to gain an understand of the rules of expert evidence.