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article / July 30, 2020

Kelly-Frye, Daubert, Mohan, and why you need to understand these cases


In the world of video-centric investigations and court cases, it is common to hear comments and questions regarding Kelly-Frye, Daubert and Mohan challenges.

Axon Investigate is a solution built by a team of certified, forensic video analysts and helps to ensure that investigators view original video evidence and share accurate copies ready for court. Several of our staff members who work on Axon Investigate have extensive courtroom experience, and so users sometimes ask us what Kelly-Frye, Daubert and Mohan challenges mean in regard to legal challenges of video evidence.

Disclaimer: The information contained in this post is for informational purposes only, and is not a statement of law. This information is not intended to provide legal advice and should not be interpreted such. Readers should always confer with an attorney to obtain legal advice and consult with local, state, and federal ordinances and laws in their applicable jurisdiction. Our goal for this post is to provide an expert witness perspective and to summarize how our field is measured when we testify using commonly accepted methods, techniques, and tools (such as those available within Axon Investigate).

Table of Contents:

  1. Introduction

  2. R v. Mohan

  3. Kelly-Frye Rule

  4. Daubert v. Merrell Dow Pharmaceuticals

  5. Summary


It is well established that judges have wide discretion in what they allow as expert testimony. But even a judge has to interpret case law to inform his or her decision on what is permissible, relevant, helpful and not overly prejudicial.

We generally rely on three specific cases for the US and one case for Canada. The first is Frye, a 1923 case which is the formation for the case law. The Frye decision was later affirmed and expanded by the 1976 Kelly decision.

Daubert followed and, for a while, replaced the Kelly-Frye rule by defining that the Federal Rules of Evidence, not Frye, provide the standard for admitting expert scientific testimony in a federal US trial.

In Canada, most challenges to expert evidence will refer to R. v. Mohan for testing purposes.

We will dive into each case as they are all very relevant to the evolution of video science. As we cover these cases, we should point out that a lot has been written on each case, some references are deeply detailed. In this blog, references to the case law are provided where applicable.

R v. Mohan

In R v. Mohan, we explore a criminal case in Canada where Mohan, a pediatrician, was charged with four counts of sexual assault on four of his female patients, who were between the ages of 13 to 16. The defense planned to call a psychiatrist who would testify that the person who would have committed these crimes would be part of a “limited and unusual group of individuals and that the accused did not fall within that narrow class, because he did not possess the characteristics of persons belonging to that group.”

As part of the testimony, the psychiatrist, Dr. Hill was going to testify that Dr. Mohan did not fit into any of the limited groups of people that would be required in order to commit all four crimes. Dr. Hill claimed that in order to commit these crimes one would have to be either a pedophile or a sexual psychopath. He believed that Dr. Mohan did not have the characteristics of either of these limited groups and therefore could not be the person in question.

“The trial judge ruled that the evidence was not admissible, stating that it would be “merely character evidence of a type that is inadmissible as going beyond the evidence of general reputation, and does not fall within the proper sphere of expert evidence”(Ref. 4, p 11).

The Ontario Court of Appeal reversed the decision. In the appellate court’s decision, the court felt that allowing expert testimony in a criminal case to show whether an individual belonged to a select group, based on traits, should help the trier of fact to make an informed decision. Therefore, the trial judge erred in rejecting this scientific testimony.

If the appeal had stopped there, our story would be over and this probably would not be a litmus test for video evidence, but the case was further evaluated by the Canadian Supreme Court. Interestingly, the higher court reversed the appellate court’s finding.

In a ruling written by Justice John Sopinka, he outlined that admissibility of an expert is controlled by four factors:

  • relevance,

  • necessity in assisting the trier of fact,

  • absence of any exclusionary rule (a law that prohibits the use of illegally obtained evidence in a criminal trial),

  • the proper qualification of the expert.

He went on to highlight that expert evidence should not be used if the potential to prejudice the trier of fact outweighs the probative value. Further, expert testimony should be avoided if it will distort the fact-finding process.

As many Axon Investigate users are based in the US and in Canada, it has been interesting for us to see how the legal interpretations extend beyond our respective borders. Since the Mohan case was focused on a phycological evaluation, the American Academy of Psychiatry and the Law discussed the implications of Daubert and Frye as it relates to the weight that was placed on Mohan.

Understanding the influences and the thought processes behind these critical cases is extremely fascinating and widely acknowledged, as outlined by the publication The Admissibility of Expert Evidence.“ Recent decisions in Canadian Law suggest that it is evolving in a manner heavily influenced by American law. A recent Supreme Court decision uses the framework of prevailing law and superimposes the more stringent criteria enunciated in Daubert v. Merrell Dow Pharmaceuticals, Inc.

Kelly-Frye Rule

So, let’s dig into the case that started it all: Frye. According to Cornell University, the Frye rule is defined as the “standard used to determine the admissibility of an expert’s scientific testimony, established in Frye v. United States, 293 F. 1013 (D.C. Cir. 1923). A court applying the Frye standard must determine whether or not the method by which that evidence was obtained was generally accepted by experts in the particular field in which it belongs. The Frye standard has been abandoned by many states and the federal courts in favor of the Daubert standard, but it is still law in some states.”

The Frye decision has been the standard case law measurement for nearly 100 years. In 1923, the Defense tried to have the scientific equivalent to a polygraph machine entered into evidence. This technology would measure blood pressure during questioning, and a trained professional would use the provided information to assess if someone was lying.

This was based on the assumption that truth required no thought, whereas lying required the brain to think and ultimately increase the blood flow and heart rate, thus driving blood pressure higher than the resting average.

Unfortunately for Frye, he lost his appeal. As the gatekeeper of evidence, the judge determined that the blood pressure method was not generally accepted, and he rejected the expert witness’s evidence. This concept of expert witness testimony requiring “general acceptance” in the scientific community was pretty much the standard by which all scientific evidence was measured until 1976.

In 1976, Frye was further reaffirmed and expanded based on The People v. Robert Emmett Kelly. Docket No. Crim. 19028. Supreme Court of California. The result of this decision is a legal test often referred to as the Kelly-Frye rule.

At issue in this case, was an emerging technique described as “voiceprint”, which involved a spectrographic analysis comparing a voice on a control tape to a voice recorded on the evidence tape to determine if there was a match.

After a series of anonymous, threatening telephone calls to Terry Waskin, Kelly was convicted of extortion.

Working with Waskin, police officers managed to record two of the extortion calls to tape. They also recorded a telephone call with Kelly to serve as the control tape. Lieutenant Ernest Nash of the Michigan State Police then performed a spectrographic analysis of all the tapes and formed the opinion that the voices on the tapes were those of the same person.

Unfortunately for the People, the appellate court found three critical problems with Nash’s testimony which were cause for Nash’s evidence to be excluded and for Kelly’s conviction to be reversed. The first issue was whether or not the testimony of a single witness is “sufficient to represent, or attest to, the views of an entire scientific community regarding the reliability of a new technique.”

Secondly, the court questioned Nash’s impartiality as he was associated with developing and promoting the technology. “Nash, a strong advocate of the voiceprint technique, may be too closely identified with the endorsement of voiceprint analysis to assess fairly and impartially the nature and extent of any opposing scientific views. A more detached and neutral observer might more fairly do so.”

Finally, the court objected to Nash’s academic qualifications to express an expert opinion in the field of voiceprint analysis that represented the view of the scientific community. The training and experience Nash cited, while impressive, represented the work of a technician rather than a scientist.

The court found this field to “be one in which only another scientist, in regular communication with other colleagues in the field, is competent to express such an opinion.”

Daubert v. Merrell Dow Pharmaceuticals

The Kelly-Frye Rule was the law of the land until the 1993 Daubert case which found that expert scientific testimony admitted in a federal trial must meet the standard of the Federal Rules of Evidence, not Frye.

“The Rules… place appropriate limits on the admissibility of purportedly scientific evidence by assigning to the trial judge the task of ensuring that an expert’s testimony both rests on a reliable foundation and is relevant to the task at hand.”

While considering a Daubert challenge, the trial judge takes the role of the ‘Gate Keeper’, protecting the trial record from unreliable, unhelpful, or prejudicial evidence.

In Daubert v. Merrell Dow Pharmaceuticals, Inc., petitioners alleged their birth defects were caused by a prescription drug, despite the studies statistically concluding that the drug did not cause birth defects. The petitioners provided eight highly credentialed experts who countered this opinion through animal studies, chemical structure analyses, and the unpublished “reanalysis” of previously published human statistical studies. However, the court ruled this evidence was not admissible because it did not meet “general acceptance” standards for expert testimony as required under Frye.

Upon appeal to the Supreme Court, the decision was vacated and remanded back to the lower court based on Evidentiary Rule 702 which states:

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.

Based on the court’s opinion, the key requirements of Rule 702 are evidentiary reliability and relevancy. A judge must assess if the proposed methodology of the testimony is scientifically valid and if the scientific knowledge will assist the trier of fact in understanding or determining a fact in question.

While Daubert’s intention was to give the judge more flexibility in allowing expert witnesses, the opposite often occurs. In fact, the Florida Supreme Court ruled in 2017 that certain “amendments to the Florida Evidence Code codifying Daubert were unconstitutional and infringed on the Court’s rulemaking authority.”

The majority’s opinion stated:

“Despite the Supreme Court’s intention that Daubert be applied flexibly, it has been observed that, in actuality, the gatekeeping role bestowed upon the judiciary has blocked more court access than it has enabled … defendants often exploit the requirements of Daubert as a sword against plaintiffs’ attorneys. Others have written that Daubert has “produced a minefield clogged with ‘Daubert hearings’ that are more lengthy, technical, and diffuse than anything that preceded them.”


For our Axon Investigate users testifying in the US, it can be helpful to review your state’s criteria for challenging expert witness testimony. Many states have adopted Daubert in its’s entirety or with some revisions, some states use the Kelly-Frye standard, while still other states have adopted their own standards that are loosely related to the ones detailed in this blog.

While these cases can create challenges for the Forensic Video Analyst, the court will continue to weigh questions of reliability, general acceptance, prejudice, and other factors against the opinions of an expert. In many cases, the courts will also have to consider opposing “experts” who may have conflicting interpretations of the same data. For those of you interested in seeing how two experts can view the same data differently, check out Exhibit A, Episode 1 (2019) on Netflix.

The episode is about a Texas man who was convicted of robbery and sentenced to twenty-eight years in prison. The program details the murky forensic video evidence that leads to his conviction and the evidence of Grant Fredericks during an appeal, which helped secure his release ten years later.

Join us next for a deep dive into a recent case that went to the Court of Appeal, Fourth Appellate District, Division One, State of California, The People v. Hung Tran where video evidence was aggressively challenged by the Defense under Kelly-Frye. The appellate court’s ruling is a big win for the admissibility of a tried and tested technique available to all video analysts.

Learn more about Axon Investigate, or our related digital evidence solutions like Axon Evidence and Axon Justice.