When I teach my Courtroom Testimony for Expert Witnesses course, a topic that frequently arises is how the decision is made as to whether to present an expert as an expert witness or a lay witness at trial. The decision may have significant ramifications and the purpose of this article is to explore that topic in more detail. I will first discuss the differences between an expert witness and a lay witness. Second, I will outline the decision-making process involved in choosing which avenue to follow when presenting the testimony of an expert. Finally, I will discuss the impact of that choice on the expert when testifying. 

What is an Expert Witness?

An expert witness is someone with specialized knowledge in a particular area of expertise who applies that knowledge in a judicial setting. Practically, what sets experts apart from lay witnesses is that while both types of witness can provide the court with factual data, only the expert witness is permit toed to evaluate the data and render an opinion as to what it means. I will refer the “lay” witness as that is the term used in the jurisprudence, although “fact” witness is also a colloquially used term. Expert witnesses assist the trier of fact in drawing inferences in areas where the expert has relevant knowledge and experience beyond that of a lay person. Opinions and conclusions presented by expert witnesses are restricted to the subject matter of the expert’s stated field of expertise. 

The parameters of expert witness testimony in U.S. federal prosecutions are set out in Federal Rule of Evidence 702 as follows:

Testimony by Expert Witnesses

A witness who is qualified as an expert by knowledge, skill, experience, training, or education may testify in the form of an opinion or otherwise if:

(a) the expert’s scientific, technical, or other specialized knowledge will help the trier of fact to understand the evidence or to determine a fact in issue;

(b) the testimony is based on sufficient facts or data;

(c) the testimony is the product of reliable principles and methods; and

(d) the expert has reliably applied the principles and methods to the facts of the case.

Lay witness testimony is covered by Rule 701 as follows:

Opinion Testimony by Lay Witnesses

If a witness is not testifying as an expert, testimony in the form of an opinion is limited to one that is:

(a) rationally based on the witness’s perception;

(b) helpful to clearly understanding the witness’s testimony or to determining a fact in issue; and

(c) not based on scientific, technical, or other specialized knowledge within the scope of Rule 702.

It is therefore the expert’s scientific, technical, or other specialized knowledge that sets the expert witness apart from the lay witness and justifies a broader opinion repertoire on the witness stand. Provisions akin to FRE 701 and 702 are found in state codes of evidence and govern prosecutions at the state level. There remains a further overriding test that is used to assess whether expert evidence will be admissible. Even if the evidence is admissible via FRE 702, it must also withstand scrutiny under FRE 403:

The court may exclude relevant evidence if its probative value is substantially outweighed by a danger of one or more of the following: unfair prejudice, confusing the issues, misleading the jury, undue delay, wasting time, or needlessly presenting cumulative evidence.

This overriding test applies to all evidence and is frequently argued in the context of expert evidence that otherwise passes admissibility tests. 

In Canada, the admissibility of expert witness testimony is governed by case law rather than codified rules. Admissibility is contingent upon establishing that the proposed expert evidence is relevant to the issues being litigated, that it is necessary to assist the trier of fact, that the area of expertise is not subject to an exclusionary rule, and that the expert is properly qualified (White Burgess Langille Inman v. Abbott and Haliburton Co., 2015 SCC 23 (SCC); R. v. Mohan, [1994] 2 S.C.R. 9 (SCC)). Proposed expert evidence that meets these mandatory requirements must then undergo a further assessment of probative value versus prejudicial effect, taking into account concerns of undue usage of time not commensurate with the value of the evidence and the likelihood of misleading the trier of fact. These additional steps are similar to those addressed by FRE 403 and equivalent state provisions. In White Burgess, the Supreme Court of Canada noted a jurisprudential trend in Canada towards tightening admissibility requirements and enhancing the trial judge’s gatekeeping role. 

To achieve the judicial designation of expert witness, the expert must be shown to have acquired specialized knowledge through targeted study and experience in respect of the matters in question that is beyond that of a lay person. Should a judge admit expert witness testimony, the trier of fact is responsible for assessing the validity and reliability of the evidence and assigning the appropriate weight to it. “Trier of fact” refers to the people who are responsible for deciding the proven facts of a case. When the trial is heard by a jury, the jury is the trier of fact. When the trial is heard by judge alone, the judge is the trier of fact. This weighing exercise is essential, especially when the expert evidence faces opposition and when competing expert evidence is presented.

Whether to Present an Expert as an Expert Witness or a Lay Witness

Counsel must decide in which capacity an expert will be used at trial as procedural steps involving notice and disclosure are triggered depending upon the decision. This decision should be made after thorough consultation with the expert about the evidence to be given. Fundamentally, this decision will be based on the evidence counsel intends to present via the expert. I will begin with this proposition – an expert need not be called as an expert witness solely by virtue of being an expert. The need to qualify the expert as an expert witness is solely dependent upon the evidence to be given. Here are the scenarios to be considered:

a) The expert gives factual evidence and expresses no expert opinions 

Many experts present evidence that is factual and does not require the expert to offer opinions about those facts in the context of the case. In these circumstances, there is no need to go through the additional procedural hurdles to qualify the expert as an expert witness as it will make no difference to the evidence presented to the trier of fact. 

There are many examples that may fall under this category, though I will mention only three. First, a forensic video expert who exports or receives, processes, and presents video evidence but does not interpret the images or offer opinions on their authenticity or content identification. As no opinions are being offered, there is no need to qualify the expert as an expert witness. Second, a computer forensics expert who discovers files and images on a device and presents those findings at trial or is asked to explain how certain computer programs or applications work. Strictly speaking, the expert is presenting factual findings about the data recovered and the programs or applications involved and is not offering opinions on the data itself. If the expert is asked to interpret the meaning of the data, that enters the expert evidence realm, and the expert must be qualified as an expert witness. Third, a crime scene expert who testifies about things that were found at a scene without interpreting the meaning of those items. Central to these examples is the fact that the expert is indeed undertaking expert work when gathering the evidence, but the expert is testifying as to facts, not interpreting the evidence, or opining on its significance. In these circumstances, the expert would not need to be qualified as an expert witness.  

b) The expert gives factual evidence and expert opinions about those facts 

In these circumstances, while the expert could give factual evidence without being qualified as an expert witness, the expert would not be permitted to interpret the evidence or give considered opinions about its significance without being qualified. Most expert evidence tends to fall in this category. 

There are many examples that fit in this category. First, a forensic video expert who presents factual testimony about the recovery and processing of media for presentation in court. No expert witness designation is required for those purposes. However, if the expert is asked to interpret the images, narrate the action, comment on authenticity, engage in image comparison, or height determination, etc., that is clearly expert territory, and the expert must be qualified to give opinion evidence. Second, a computer forensics expert who is asked to explain what files were found on a device and additionally, to offer opinions about their authenticity or whether the files (think child pornography) could have been put there accidentally or by a hacker. This would require that the expert be qualified as an expert witness. Third, a crime scene expert who is asked to interpret the evidence found at a scene must be similarly qualified. The common theme in these examples is that the expert is not just giving factual evidence but is interpreting the evidence and opining about some aspect of it.  

c) The expert gives no factual evidence and only expresses expert opinions 

Some experts do not present any factual evidence that they have discovered but rather opine on evidence found by others or on hypotheticals presented to them. Examples in this category include forensic psychiatrists and psychologists who testify about a person’s state of mind and mental capacity premised on information provided to them for that purpose; a forensic toxicologist who provides a retrograde extrapolation opinion about blood alcohol content based on a hypothetical; or a peer reviewer giving an opinion as to the soundness of another expert’s work. These examples involve expert opinion evidence and the expert must be qualified accordingly. 

Why the Choice Matters

The capacity in which an expert is tendered as a witness sets the parameters for what the expert will be allowed to testify about. When presenting evidence as a lay witness, the expert will be restricted to factual evidence and will be prohibited from interpreting the evidence or offering any opinions that are properly expert opinions. This applies to answering questions by counsel for all parties. For this reason, counsel and the expert must carefully evaluate all the evidence to be presented and assess whether expert opinions are required or desirable. It sometimes occurs during testimony that counsel wishes to draw out an opinion from the expert that was not contemplated. That will not be permitted if the expert has been presented as a lay witness.  

Some attorneys prefer to limit the evidence of an expert to factual matters only and therefore decline to pursue an expert witness designation. This may be done because only factual testimony is required and, in those circumstances, that decision is defensible. Other attorneys choose not to seek an expert designation because it requires more effort, engages additional scrutiny by counsel and the court, and adds a level of complexity the attorney may prefer to avoid. However, deciding to limit the expert to factual evidence only may do harm to the case and artificially limit the expert’s scope of evidence.

During my career as a prosecutor, my practice was to qualify experts as expert witnesses so that I had the requisite latitude to go into expert opinion areas if needed. That practice avoided the situation of opposing counsel objecting to my questions on the basis that they called for an opinion. In most cases, more value will be obtained from an expert if they are qualified as an expert witness. Their testimony may also be given additional credence by the trier of fact, who during the qualification process will be fully apprised of the expert’s extensive educations, training, and experience.