Rules for admitting Expert Opinion Evidence in Criminal Trials

Expert opinion evidence is commonly presented by the Crown and defence in criminal trials — for example, testimony given by the arresting officer or toxicologist on an impaired driving charge — and such evidence often plays an important role in proving or disproving a case.  However, expert opinion evidence is viewed as presumptively inadmissible and it must adhere to specific criteria set out in case law in order to be determined admissible in court.

In R. v. Abbey (2009), the Court decided that the party presenting the expert evidence is required to establish its admissibility on the balance of probabilities. This rule applies to expert evidence presented by either the Crown or defendant.  Further, the judge must determine the scope and type of the proposed expert evidence in order to set boundaries for the application of the evidence — a judge may choose to admit part or all of the expert’s testimony; may modify the scope of the expert’s opinion; and may edit the language of the testimony to frame the expert’s opinion.

Making a determination whether expert evidence is admissible is a two-step process.  The first step was defined in R. v. Mohan (1994), where the Supreme Court of Canada set four specific criteria for the threshold admissibility of expert evidence: 1) relevance; 2) necessity (with regards to assisting the trier of fact); 3) absence of exclusionary rule; and 4) ‘properly qualified’ (i.e. having special knowledge, through experience or education, on the subject matter at issue).  Further, when an expert opinion is sought on contested or novel science, admissibility may be based on the reliability of the underlying science. 

In White Burgess Langille Inman v. Abbott and Haliburton Co. (2015), the Supreme Court of Canada set a second requirement for a ‘properly qualified’ expert witness. In addition to being qualified to give the particular opinion evidence, as defined in Mohan, a properly qualified witness must exhibit independence and impartiality, which means they must be able and willing to provide fair, objective and non-biased assistance to the court. 

In White Burgess, it was acknowledged that most experts have some connection with a party or with the litigation, but this is not sufficient reason to disqualify a proposed expert.  Whether an expert who has an interest or connection will be allowed to give evidence “is a matter of fact and degree”.   Four areas of ‘more concern’ that were identified by the Court are: 1) an expert who has a “direct financial interest in the outcome” of a trial; 2) a “very close familial relationship” with a party; 3) the likelihood of professional liability if the expert’s opinion is rejected; and 4) when an expert takes “the role of an advocate” for one of the parties. 

Witness independence is assessed subjectively on whether the witness’s relationship or interest causes the expert to be unwilling or unable to carry out their primary duty to the court, which is to provide fair, non-partisan and objective assistance.  In White Burgess, the court stated that once the party presenting the expert has satisfied all the admissibility requirements and testified that their duty to the court is accepted, the burden shifts to the opposing party to show a ‘realistic concern’ that the expert is unwilling or unable to comply with their duty.  If the opponent is successful in doing so, the burden shifts back to the party proposing the expert, and failure to satisfy the requirement means the evidence is blemished by a lack of impartiality or independence and must therefore be excluded.

R. v. Dixon: Judge rules on admissibility of expert evidence

In R. v. Dixon, a man faced the charge of possession of a controlled substance (fentanyl) for the purpose of trafficking.  The Crown wished to present expert opinion evidence from a Chatham-Kent police officer on how fentanyl is unlawfully consumed and distributed, but the defence contested the admissibility of this particular officer’s evidence on the grounds that the evidence was unnecessary and further, the expert was not properly qualified and lacked independence and impartiality.  The issue to be decided in this case was the admissibility of the expert evidence.

The judge first applied the criteria in R. v. Mohan and found that the witness’s opinion evidence was relevant as the case concerned a trafficking charge and evidence was being sought on the distribution, packaging, prices and so on for fentanyl.  The judge found that as the consumption and sale of fentanyl was outside the Court’s knowledge, the proposed expert evidence was necessary. However, the judge stated that the Crown’s questions would be restricted to issues outside the judge’s knowledge and experience only.  The judge also found that the officer was a properly qualified expert, given his years of experience.  A remaining question addressed by the court was whether the officer was an independent and impartial expert, and willing and able to care out his primary duty to the court. 

Although the officer testified that he was able to be an independent and impartial expert, the judge noted that he was obligated to make his own assessment of the officer’s impartiality based on a number of questions, including: whether the case and investigation were major or routine, how involved was the officer in the investigation and/or arrest, and whether the officer will be called as a fact witness by the Crown.  The officer was not involved in the arrest, search and seizure, and had only a minor role in the investigation.  Also, the officer would not be permitted to give opinion evidence on the accused’s mens rea (i.e. knowledge or intention to commit trafficking).  On all these grounds, the judge decided that the Crown met its burden to show that the witness would, on a balance of probabilities, be impartial and independent.

Finally, the judge weighed the benefit and cost of receiving the expert evidence from the witness. The judge found there was a significant benefit as the evidence was important, although acknowledging that the witness did have some connection to the case.  The potential cost was low as there was no significant time and no complication, prejudice or confusion.  The judge concluded that this was not a case where the proposed expert evidence must be excluded.

At the Law Firm of Ted Yoannou, our criminal law team boasts many years experience in successfully defending persons accused of a criminal offence, such as drug trafficking or drinking and driving.  If you have been detained or charged, don’t hesitate to consult with one of our expert staff to ensure that your case receives the best possible outcome and is resolved as quickly as possible. 


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