As a general rule of evidence, witnesses are not permitted to provide testimony on their opinion. Instead, they may only testify based on observations they themselves made. Inferences cannot be made. An exception to this rule is expert witnesses.
Expert witnesses are permitted to provide opinion evidence to assist the court in understanding a particular issue or topic. Expert opinions can only be utilized when it is necessary to do so. It is considered necessary when the topic or issue is one which the judge and/or jury is unlikely to be knowledgeable about.
What is Expert Evidence?
Expert evidence is provided by an expert witness who testifies at a criminal trial to provide an opinion on a particular topic for which they have qualified knowledge. Typically, the expert will review the facts of the case prior to trial and will prepare a report of their opinion and the reasons for that opinion. They will then testify at trial, explaining their report to the court. An expert must only provide testimony on the particular area of knowledge for which they are qualified to provide an opinion.
When can Expert Evidence be Used?
Expert evidence is used at trial when the court needs assistance in understanding a particular issue and where that information cannot be introduced in any other way, making an expert necessary. Before an expert opinion can be admitted as evidence, it must pass a two-step test. This is known as the Mohan test.
For the first party of the test, the party calling the expert witness must satisfy the court that the opinion of the expert is relevant to the issue at hand, is necessary to properly educate the trier of fact on the issue, that no other exclusionary rule of evidence applies to the evidence that will be given, and that the expert is properly qualified.
The second stage of the test requires that court weigh the benefits and risks of admitting the evidence to determine whether doing so is appropriate. This is known as the probative and prejudicial value of the evidence. If the risks outweigh the rewards, the evidence will not be admitted.
Expert Evidence and Criminal Procedure
In a criminal trial both the Crown and defence can call a witness to provide an opinion on a relevant topic. As long as the evidence passes the two-step test outlined in the prior section, the expert may testify. In some cases, the court may require the expert witness to appear before the court and be questioned on their qualifications in what is known as a voir dire.
Prior to trial, the party wishing to call the witness must provide notice to the court and to opposing counsel as outlined in section 657.3(1) of the Criminal Code. The expert must also prepare a report to be used as evidence and submit this report to the court and Crown prior to trial.
When an expert witness is called to testify, they will take the stand and be sworn in. The party calling the witness will then begin with their examination in chief of the expert. Through the examination in chief, the expert will present their findings and opinion to the court. Once the examination in chief has concluded, the opposing party will have an opportunity to cross-examine the witness. During cross-examination the lawyer may ask about anything discussed during examination in chief.
Who is Qualified to be an Expert in a Criminal Case?
To determine whether a particular individual is qualified to be an expert in a criminal matter, the expert must demonstrate that they have a specific set of knowledge that they have acquired through study or experience in the particular area they are providing evidence on.
An expert witness must be unbiased and objective in their opinion and testimony. Their job is not to argue for one party or the other. Their job is to simply provide their qualified opinion based on the facts of the case and their knowledge. Whether or not the expert is impartial will be considered when determining whether they are qualified.
When providing notice that an expert witness will be called at trial, the party must include the name of the proposed expert, their qualifications, and a description of the area of expertise they will be testifying about.
Why is Expert Testimony sometimes Necessary?
Expert testimony is necessary where the trier of fact does not have enough knowledge of a particular topic or issue to make a determination without assistance. Expert witnesses are often used in cases involving complex issues that would not be considered common knowledge. For example, in a murder trial it may be necessary for an expert witness to testify on the cause and manner of death.
What types of Criminal Cases Typically Require an Expert?
An expert witness can be called in virtually any criminal trial where their testimony is necessary to educate the court on a particular topic or issue.
Expert witnesses are often called in cases involving murders where a coroner or other medical professional would be required to explain the nature and consequences of the injuries to the victim.
Expert witnesses may also be called in cases of sexual assault where the complainant was highly intoxicated. A toxicologist may be required to present an opinion on how intoxicated the complainant was at the time based on how much alcohol they consumed and other factors. They may also provide an opinion on how that level of intoxication would affect a person.
In cases involving fraud or money laundering offences, a forensic accountant may be called as an expert witness to explain various financial matters to the court.
Are Police Considered Expert Witnesses?
In most cases, an officer who is testifying in a criminal trial is not testifying as an expert witness. Instead, they are testifying as a regular witness, like anyone else. When an officer is testifying about something they have personal knowledge of, they are considered a regular witness. For example, an officer who arrested an individual for impaired driving will testify as a regular witness because they have personal knowledge of the events surrounding the accused arrest.
A police officer may testify as an expert where they do not have personal knowledge of the issue or topic but have a specialized knowledge related to the particular topic that may assist the court in some way.