When bringing in an expert to testify something, obviously that expert doesn't have to bring in their resume and go through 3 rounds of interviewing with every member of the jury, as if they were trying to get hired at Apple.

So what is stopping someone from bringing Dr. SnakeOil from Scam College in as their expert witness to testify something? As a simple example, even chiropractors are controversial in the medical field. If Charlie the Chiropractor comes in and says the defendant's back was in too much pain for him to jump out the window, is that word allowed in the courtroom as expert testimony? Or, as a more extreme example, is Wally the Witchdoctor allowed to come in and say the same thing? Ideally, one would have Sam the Stanford M.D. make the statement, of course.

Basically, what is the standard of quality for an expert witness, or are juror's/judges supposed to make up their own minds based on short descriptions (e.g. they graduated from Harvard, they've treated 1000 patients, etc.)? But that also seemingly leads to problems. Not everyone will know that UCSF is the best medical school in the world (I'm showing my personal bias here xP), or that Donda academy isn't accredited, etc..

  • Broadly, a witness is accepted as an expert, or not. If that point can to judgement, would it not boil down to criteria similar to what the expert might have faced in his or her job interview? Commented Nov 6, 2022 at 23:21
  • "obviously that expert doesn't have to bring in their resume" Actually, they almost always do have to bring a resume, and it is pretty much required as a consequence of Fed. R. Civ. Pro. 26(a)(2) and Federal Rule of Evidence 702 in federal court, and by parallel rules in state court, and there is an interview by opposing counsel and sometimes the judge (called voire dire if it happens at trial and a Daubert hearing if it happens before trial). The standard is not necessarily the same in state court and in federal court, even though the outlines of the process are usually similar.
    – ohwilleke
    Commented Nov 7, 2022 at 4:46
  • Tongue-in-cheek: The one whose opinion suits me is the most qualified. Commented Nov 7, 2022 at 9:14
  • I think this is as good a time as any to pull out "My Cousin Vinny" and the expert testimony scene: youtube.com/watch?v=3nGQLQF1b6I - it even has an invitation to "voire dire" the witness. :)
    – Frodyne
    Commented Nov 7, 2022 at 10:07

3 Answers 3


Qualifed expert

Experts do have to essentially "bring in their resume." For example, in Ontario, an expert's report must outline "the expert’s qualifications and employment and educational experiences in his or her area of expertise." Rules of Civil Procedure, 53.03.

An expert must "have acquired special or peculiar knowledge through study or experience in respect of the matters on which he or she undertakes to testify." R. v. Mohan, [1994] 2 S.C.R. 9.

The risks

The Supreme Court has recognized (Mohan):

There is a danger that expert evidence will be misused and will distort the fact-finding process. Dressed up in scientific language which the jury does not easily understand and submitted through a witness of impressive antecedents, this evidence is apt to be accepted by the jury as being virtually infallible and as having more weight than it deserves.


The cases address a number of other related concerns: the potential prejudice created by the expert’s reliance on unproven material not subject to cross-examination; the risk of admitting “junk science”; and the risk that a “contest of experts” distracts rather than assists the trier of fact. Another well-known danger associated with the admissibility of expert evidence is that it may lead to an inordinate expenditure of time and money. [Citations removed]

The gatekeeping role of the judge

The judge is tasked with a "gatekeeping role": "the judge balances the potential risks and benefits of admitting the evidence in order to decide whether the potential benefits justify the risks."

The judge is to compare the potential value of the expert evidence with its potential risks. In doing so, the judge looks at (R. v. J.-L.J., 2000 SCC 51):

  • the subject matter
  • whether this is a scientific question
  • whether the expert evidence is on the ultimate issue before the court
  • whether the evidence would be precluded by another exclusionary rule
  • whether the expert is properly qualified
  • the relevance of the evidence
  • the necessity of the evidence

If the question is a scientific question, Canada has adopted an approach similar to that used in the United States that asks (R. v. J.‑L.J., 2000 SCC 51; R. v. Trochym, 2007 SCC 6):

  1. whether the theory or technique can be and has been tested
  2. whether the theory or technique has been subjected to peer review and publication
  3. the known or potential rate of error or the existence of standards
  4. whether the theory or technique used has been generally accepted

Evidence admitted is not evidence accepted

If the judge is of the view that the expert evidence is admissible, then it is up to the opposing side to introduce competing evidence regarding the question at issue (even using their own experts) and can cross-examine the expert on their qualifications or the scientific bases for their opinions.


In the US, there is more reliance on the evidence to be introduced compared to the credentials of the person testifying. The most widely-adopted evidentiary standard is the Daubert Standard. This translates into a series of rules in Articles I and VII of the Federal Rules of Evidence, adopted by most states, which requires the trial judge to determine if the testimony is relevant and rests on a reliable foundation. Rule 104(b) says that

When the relevance of evidence depends on whether a fact exists, proof must be introduced sufficient to support a finding that the fact does exist. The court may admit the proposed evidence on the condition that the proof be introduced later.

In other words, it could be a legally relevant question whether a substance is cocaine, or baking powder. The party wishing to exploit a particular fact must establish to the court's satisfaction the relevance of the purported fact. Then FRE 702 governs qualifications:

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.

More emphasis is placed on objective properties of the purported knowledge, compared to the training of the expert. Indeed, the requirement for the expert is not about training, it is more broadly about knowledge, skill, experience, training, or education.

The scientific bar that has to be cleared for admissibility is not particularly onerous. So-called "voice prints" are admissible in many jurisdictions, though in the domain of academic acoustic experts there is a significant concern that voice prints have not been "calibrated" i.e. objectively proven to be reliable (see United States v. Addison, 498 F.2d 741). The primary question, then, is whether this kind of evidence is relevant and reliable, and the qualifications of the person doing the measurement is less important (therefore, an FBI lab technician can testify to one fact, and an internationally-recognized professor of speech acoustics can testify to an opposing fact – the jury has to sort it out).

Credentials were a central issue in Kumho Tire Co. v. Carmichael, 526 U.S. 137, where one of the holdings was that "The Daubert factors may apply to the testimony of engineers and other experts who are not scientists", "It is the Rule's word "knowledge," not the words (like "scientific") that modify that word, that establishes a standard of evidentiary reliability". The chiropractor could therefore be allowed to testify to the back-pain question, and a sports-medicine MD could likewise testify to the opposite fact.


There was an infamous case in the UK. Some medical doctor claimed that if a baby died for no visible reason, he could interview the mother and after interviewing her he would know whether this was a case of "sudden cot death" as a murder. What qualifications? He had done it before and he was always right, because he always agreed with his own opinion. If you think that he was basically totally unqualified, you would be absolutely right.

He put several mothers in jail until he was struck off. Lookup Roy Meadows.

  • I suppose there's no systemic solution to such a problem, yeah? All one can hope for is to hire strong (and expensive) lawyers who would set the record straight with such "experts"?
    – chausies
    Commented Nov 6, 2022 at 9:55
  • 3
    Well, the lawyer would have to be an expert himself. For example, Roy Meadows used some fatally flawed statistics. I would have recognised it after taking my first or second semester of statistics while studying maths. An excellent lawyer might never learn about it. (Actually one statistics professor of mine went to court as expert witness, testifying that there is no system that allows you to win at roulette through statistics. )
    – gnasher729
    Commented Nov 6, 2022 at 10:23

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