Testifying as an Expert

Henry Waldock
Last updated:   2018.10.14


Courts do not require witnesses to collect university degrees before testifying as experts.  Anyone who possesses special knowledge and experience going beyond that of ordinary people is capable of giving expert opinion evidence.  Formal university qualifications add to credibility, but are not required.

However, expertise must be proved.  Education, training, research and experience all help show what special knowledge you have that the rest of us don't.  You may be required to explain the credibility of the teachers, textbooks or publications from which you obtained your training.

Even if you are an expert, courts won't always permit you to testify.  The expertise must be:

In reports and in testimony, an expert witness must be fair, impartial and objective.

Expert's Qualifications

If you know that you are going give an expert opinion, the court needs to know how you got to know more on the topic than ordinary people.  Prepare and maintain a curriculum vitae.  It should:

research and experiments
articles written
positions or appointments
courses taken
courses taught
membership in professional societies
special achievements in the field
personal contact information you don't want the defendant to have
long paragraphs under each heading
high school graduation

Counsel disagree whether it should list the courts or cases in which you have been accepted as an expert.  I quite like seeing this on C.V.'s.  It helps me know how much preparation the witness needs.  But I don't like long lists of cases.

Don't worry if it seems short.  The best qualified experts I have ever presented kept their C.V.s down to 2-3 pages by leaving out minor or unrelated information.

Send a copy of your C.V. with your report, and bring several copies to court.

Area of Expertise

At court, the lawyer tendering your evidence will need a "sound-bite" description of your expertise.  The lawyer will appreciate it if you can offer one. Here are some examples.  Don't feel bound to them.  Your opinion may rely upon greater or different expertise than these examples:

"I am an expert in ...
Accident Reconstruction
... identification and interpretation of skid marks on roads, and the calculation of the speeds and dynamics of the vehicles that made them."
... the theory and practice of tracking human scents with police dogs, and
Police dog _____ is a trained and expert tracker."
Fingerprint Examiner
... finding, preserving, interpreting, comparing and identifying friction ridge impressions, including fingerprints and palmprints."

Some examples of unusual expertise that courts have accepted include:

A controversial area of expertise is opinion evidence whether a crime scene was "staged" in order to divert suspicion from the true perpetrator. The trouble is that the expert testifies based on what offenders "usually do", which constitutes psychological profiling.  There's not enough science to back this up. R. v. Ranger 2003 CanLII 32900, 178 C.C.C. (3d) 375 (Ont. C.A.), and R. v. Clark 2004 CanLII 12038 (ON C.A.).  In R. v. Klymchuk, 2008 CanLII 23495 (ON S.C.), the court accepted that this might be a useful investigative tool, but rejected it as evidence before the jury, particularly fearing that T.V. shows like C.S.I. might cause the jury to give it too much weight.  Similarly, in R. v. Stobbe, 2012 MBQB 78, the court found that the expert simply provided the same inferences a jury could draw on its own; therefore, the expert did not assist the court.  See also Murphy, 2014 YKCA 7.

Similarly, Mr Sekhon, 2014 SCC 15 drove a large load of drugs across the border.  A police expert testified he had never in 1,000 drug investigations encountered a "blind courier" - a driver who didn't know that his load carried drugs.  This too relied upon what offenders usually do (or know) to persuade the judge of his guilt rather than examining what this offender did (or knew).  The judges of the Supreme Court unanimously rejected this evidence.

So don't give expert opinions about what criminals usually do unless very specifically asked.  On the other hand, make a list of all the observations that made you think this, and make sure you tell the jury every item on your list.  Let counsel draw all those observations into a logical conclusion.

Some opinions aren't really expert evidence at all, but common-sense inferences from common experiences.

Voir Dire to determine Expertise

The prosecutor will introduce you to the court as an expert, using the "sound bite" described above.  Often, defence admits your expertise.  If not, the prosecutor will ask you:

The prosecutor will file a copy of your c.v. with the court.

Defence may cross-examine to show that you do not have the expertise you claim, or to show that your field of special knowledge is narrow.  Although it's tactically unwise, some counsel will even cross-examine you generally on the opinion in your report.


As a police officer witness, it was never your job to ensure that one side or the other "wins".  Instead, you must convey accurately what you know about the case to the judge (or jury).  As an expert, this principle applies with even greater force.  Indeed, if you try too hard to win the case, you may destroy your credibility, and wind up losing the case instead.  R. v. Proctor, 2008 BCSC 19 (B.C.S.C.).  In some provinces, the rules for civil procedure codify this duty of impartiality.  (British Columbia, Ontario)

In White Burgess Langille Inman v. Abbott and Haliburton Co., 2015 SCC 23, the court held that judges should generally not let experts testify unless the expert understands his or her duty "to be fair, objective and non-partisan", and testifies to that effect when being qualified. They gave this direction in a civil case. In civil litigation, the parties choose experts who are most likely to assist their side, which tends to lead to biased and partial experts.

The absence of such testimony caught the court's attention when a prosecutor's wife tendered expert evidence about her analysis of digital security video. Defence cross-examination showed that she gave a seminar emphasizing her value as a prosecution witness. She looked biased. Smith-Wilson, 2016 SKQB 33.


Don't fudge your results. It's not professional, and can cause untold costs and grief. In Abbey, 2017 ONCA 640, an expert claimed to have strong evidence about the meaning of teardrop tattoos among gang members. Perhaps he did have special knowledge, but he fudged the statistics he offered to support this opinion. Lawyers eventually figured it out, destroying his reputation. But not before he cost the public masses of litigation costs. Now, his evidence is worth nothing. It is possible that a murderer will walk free, or that an innocent man suffered a decade of litigation.


Because of the institutional sympathy between police expert and police investigator, the police expert is vulnerable to accusations of bias against the suspect. This becomes particularly acute if the police expert participates in the investigation of the suspect and then later offers himself as an "independent" expert. McManus, 2017 ONCA 188.

As much as possible, police experts should try to keep themselves separate from the investigations on which they give opinions.

Expert's Report

Data base: Your report should set out what relevant information you obtained about the case.  This may be:
- hearsay (such as eyewitness statements to an accident or photographs that others took)
- personal observations (the accident reconstructionist's observations and measurements of a collision scene or a dogmaster's observations of the dog's actions while following a track).

Role:  Experts must present fair, objective and non-partisan opinions, and must provide opinion evidence only in areas within the expertís area of expertise.  

Ontario's civil rule 4.1.01 imposes this duty on experts in Ontario's Superior and Appeal courts.

British Columbia's Supreme Court Civil Rules requires experts to state their duties in their reports in the following language:
"Rule 11-2 of the British Columbia Supreme Court Rules places a duty on experts to assist the court, and not act as an advocate for any party.  I am aware of that duty, I prepared this report, and, if required to testify, will testify, in conformity with that duty."

In White Burgess Langille Inman v. Abbott and Haliburton Co., 2015 SCC 23, the court suggested that experts should testify in these terms during the voir dire before a court should accept them as experts.

This suggests that even for criminal cases, your report should include a similar assertion:

"I understand that the law requires me as an expert witness to present fair, objective and non-partisan opinions, and to provide opinion evidence only in areas within my area of expertise."

However, in crimnal trials, the report rarely becomes an exhibit. Your curriculum vitae usually does. You may be wise to add it there too.

A British publication explains some specific rules that apply to that country, but the "General Requirements" at Part 3 apply in Canada too. Assumptions: The report should state all assumptions on which the expert relied before reaching a conclusion.  For example:

Analysis: The report should explain the analysis, experience or logic applied to this information, so that others can understand how you reach your conclusions.  Diagrammes, calculations and photographs help tremendously.

Conclusions: The report should clearly state conclusions about what you do and do not know.  Use language understandable by a non-expert, but stated sufficiently precisely that it can not be misinterpreted.

For example:

A fingerprint examiner might conclude: The thumb and left index finger of [the known person] held the barrel of the revolver near the muzzle.  The orientation of the prints and the smudge marks suggest that he held it with just those two fingers, leaving the handle dangling down.  However, there are other possible explanations for the orientation and smudge marks.  I can not determine when the the prints were made.

An accident reconstructionist might say: The red Honda was travelling at a minimum speed of 121.8 km/h before the tires started to skid.

Your report:

Disclosing Drafts

Generally, you don't need to save or disclose every draft you make of your report. But if you give your report to a peer to review, and the peer suggests changes, then that draft should be saved, and disclosed to defence. Natsis, 2018 ONCA 425

Testifying as an Expert

Bring spare copies of your C.V. and report to court.  Counsel often forget how many copies are needed.  You look more professional if you have a spare available for the judge.

Because judges and juries don't have the training, knowledge and experience that you have, you should:

Often during testimony, experts are tempted to venture opinions that extend beyond your expertise.  The Crown always wants evidence that makes the case perfect.  Defence will challenge you or tempt you to assert what seem to you to be obvious or logical extensions of your opinion.  Beware.  It is far safer to assert "I don't have any special expertise with respect to ..." than to be exposed as a fraud, or contradicted by a real expert in the area.

Avoid expressing an opinion of the accused's guilt.  Judges guard their role as decision-makers in trials jealously.  The judge or the jury is the one who examines the evidence, draws inferences, and concludes guilt.  Therefore, when testifying, you should avoid categorical remarks like "In my opinion, the accused is guilty".  Instead, you should say: "The fingerprint I saw on the window matched the one I saw on the arrest form" or "Because conditions were good for tracking, and because of the behaviour of my police dog, I believe that the accused, who I found behind a bush, had recently gone there directly from where the dog first started tracking."

Over the last decade, lawyers everywhere debated whether expert evidence must be provable and testable according to scientific method.  Because this issue is not yet fully resolved, you should be clear about the difference between opinion based upon experience, and results based upon applied science.

Giving the Evidence

When giving your testimony:


"Yes, if the accused drank alcohol just before driving, his blood-alcohol level could rise, even after being stopped by police.  But to account for the difference between these breath tests and .08, he would have had to consume 10 beer in a 30 minute period.  This would be a highly unusual drinking pattern, and I would have expected to see the symptoms of intoxication to increase markedly from the time of the traffic stop to the time of the breath samples."

You can not tell the court whether a witness is credible, but, for the purposes of your opinion you may say which evidence you trusted and why.  For example, a ballistics expert might say:

"The victim said many shots were fired at him.  But he was woken from his sleep.  The neighbor said that she was awake, and she heard only one shot fired.  Because her bedroom window was open, and only 40 feet from victim's window, I took her version to be accurate.  If only one shot was fired, then the bullet that broke the victim's window is the one that smashed the glass by the victim's bed."

Deadlines, Disclosure and Document notice

Section 657.3 of the Criminal Code requires the prosecution to give at least 30 days notice to the defence of the expert, his or her expertise, and the material the expert will cover.  Defence also has some obligations to give notice.

For counsel to comply with these obligations, they need your C.V. and a written version of your opinion long before trial.  Please don't wait until the deadline, because last-minute scrambles create too many opportunities for mistakes.

Crown's disclosure obligations require police experts to disclose all relevant information in the possession of the police.  Usually the report will cover this, but not always.


If your opinion depends in whole or in part from information you received from other police officers or witnesses, then the court may ignore your opinion unless those other officers or witnesses testifies to the accuracy of the information you received.

For example, an accident reconstruction expert may reach the scene of a collision until after the wrecker removed the vehicles.  An officer at the scene may point out the locations where the vehicles were found.  Based upon that information, and the skid marks that the officer finds on the road, the expert may calculate the speeds of the respective vehicles.  The expert knows about the skid marks from personal observation, but the locations of the vehicles is hearsay.

The court will allow the expert to testify about what the other officer said about the locations of the vehicles, but only for the purpose of explaining the opinion.  Counsel must ask the other officer to testify in order to "establish" the facts on which the expert relies.  R. v. Scardino (1991), 6 C.R. (4th) 146 (Ont. C.A.).

Therefore, you report must identify this information as important to your opinion.

If your work was peer-reviewed, you may mention that fact in passing.  Don't dwell on it, unless specifically asked. R. v. Rogers, 2015 ONCA 399 at para 63-64.

Case Law

In R. v. Mohan, [1994] 2 S.C.R. 9 the court set out rules for lawyers as to what kinds of "expert" would be admitted in a trial, and for what purposes.

In R. v. D. (D.), [2000] 2 S.C.R. 275 the court limited the use of expert testimony on topics which didn't need it.  In that case, the court found that a jury did not need an expert to tell them that complainants who delay reporting sexual abuse are not necessarily untruthful.

Experts need not be scientists, so long as they do not present their evidence as if it were science.  Abbey, 2009 ONCA 624.

In R. v. Yeh, 2009 SKCA 112, a police officer testified that based on his training, he formed the opinion that the suspect consumed marijuana within the preceding 4 hours.  But his training didn't teach him time-lines of symptoms of drug-use.  He couldn't identify the publications he read that gave him this idea.  His expertise (though probably accurate) wasn't proved to the court's satisfaction.

For more case law in this area, I refer you to Gibson's Criminal Law,Evidence, Practice and Procedure - Chapter 31A, from which I borrowed some material for this page.  Or you could go to Watt's texton Evidence.

Someone removed glass from the rear of a building, and leaned the panes up against the building.  Someone entered through the resulting gap, and stole lots of valuable stuff.  A fingerprint examiner found Mr D.D.T.'s, 2009ONCA 918fingerprints onthe glass.  A judge concluded that sufficed to prove his guilt of the B&E and theft.  The Court of Appeal disagreed.

As usual, the expert could not say when D.D.T. touched the panes of glass.  Unusually, the expert prepared no diagramme of the locations of the prints.  Nor, it seems, did the examiner look at every print on the glass to determine if they all came from D.D.T..  He said there were many, but he matched only 7.  This gap in the evidence left the possibility that others handled the glass, and that D.D.T.'s fingerprints could have got there innocently.  A good investigator examines all of the available evidence; likewise, a good forensic examination considers all of the available information, not just the convenient stuff.

In the Goudge Inquiry, (2008) an Ontario Judge investigated a series of misdiagnoses by Dr Smith, a paediatric pathologist.  Dr Smith performed autopsies on children who died in suspicious circumstances.  He often "found" evidence of child abuse, which led to prosecutions and convictions of innocent people.  The report identified factors which resulted in bad expert evidence:

  1. Inadequate training
  2. Lack of oversight
  3. Lack of peer review - only people friendly to Dr Smith reviewed his work.
  4. Sloppy methods - poor record-keeping, poor documentation
  5. "Dirty thinking" - presuming that a crime occurred, and then looking for evidence of it.
  6. Acting as advocate for the prosecution
  7. Offering opinions outside his area of expertise
  8. Providing conclusions without explaining the thought processes
  9. Omitting, or failing to consider, evidence which contradicted the expert's theory.