Forensic Evidence Admissibility and Expert Witnesses

National Forensic Science Technology Center


How or why some scientific evidence or expert witnesses are allowed to be presented in court and some are not can be confusing to the casual observer or a layperson reading about a case in the media. However, there is significant precedent that guides the way these decisions are made. Our discussion here will briefly outline the three major sources that currently guide evidence and testimony admissibility.

The Frye Standard — Scientific Evidence and the Principle of General Acceptance

In 1923, in Frye v. United States1, the District of Columbia Court rejected the scientific validity of the lie detector (polygraph) because the technology did not have significant general acceptance at that time. The court gave a guideline for determining the admissibility of scientific examinations:

Just when a scientific principle or discovery crosses the line between the experimental and demonstrable stages is difficult to define. Somewhere in this twilight zone the evidential force of the principle must be recognized, and while the courts will go a long way in admitting experimental testimony deduced from a well-recognized scientific principle or discovery, the thing from which the deduction is made must be sufficiently established to have gained general acceptance in the particular field in which it belongs.

Essentially, to apply the "Frye Standard" a court had to decide if the procedure, technique or principles in question were generally accepted by a meaningful proportion of the relevant scientific community. This standard prevailed in the federal courts and some states for many years.

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Federal Rules of Evidence, Rule 702

In 1975, more than a half-century after Frye was decided, the Federal Rules of Evidence were adopted for litigation in federal courts. They included rules on expert testimony. Their alternative to the Frye Standard came to be used more broadly because it did not strictly require general acceptance and was seen to be more flexible.

The first version of Federal Rule of Evidence 702 provided that 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:

  1. 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;
  2. the testimony is based on sufficient facts or data;
  3. the testimony is the product of reliable principles and methods; and
  4. the expert has reliably applied the principles and methods to the facts of the case.

While the states are allowed to adopt their own rules, most have adopted or modified the Federal rules, including those covering expert testimony.

In a 1993 case, Daubert v. Merrell Dow Pharmaceuticals, Inc., the United States Supreme Court held that the Federal Rules of Evidence, and in particular Fed. R. Evid. 702, superseded Frye's "general acceptance" test.

The Daubert Standard — Court Acceptance of Expert Testimony

In Daubert and later cases2, the Court explained that the federal standard includes general acceptance, but also looks at the science and its application. Trial judges are the final arbiter or "gatekeeper" on admissibility of evidence and acceptance of a witness as an expert within their own courtrooms.

In deciding if the science and the expert in question should be permitted, the judge should consider:

  • What is the basic theory and has it been tested?
  • Are there standards controlling the technique?
  • Has the theory or technique been subjected to peer review and publication?
  • What is the known or potential error rate?
  • Is there general acceptance of the theory?
  • Has the expert adequately accounted for alternative explanations?
  • Has the expert unjustifiably extrapolated from an accepted premise to an unfounded conclusion?

The Daubert Court also observed that concerns over shaky evidence could be handled through vigorous cross-examination, presentation of contrary evidence and careful instruction on the burden of proof.

In many states, scientific expert testimony is now subject to this Daubert standard. But some states still use a modification of the Frye standard.

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Who can serve as an expert forensic science witness at court?

Over the years, evidence presented at trial has grown increasingly difficult for the average juror to understand. By calling on an expert witness who can discuss complex evidence or testing in an easy-to-understand manner, trial lawyers can better present their cases and jurors can be better equipped to weigh the evidence. But this brings up additional difficult questions. How does the court define whether a person is an expert? What qualifications must they meet to provide their opinion in a court of law?

These questions, too, are addressed in Fed. R. Evid. 702. It only allows experts "qualified … by knowledge, skill, experience, training, or education." To be considered a true expert in any field generally requires a significant level of training and experience. The various forensic disciplines follow different training plans, but most include in-house training, assessments and practical exams, and continuing education. Oral presentation practice, including moot court experience (simulated courtroom proceeding), is very helpful in preparing examiners for questioning in a trial.

Normally, the individual that issued the laboratory report would serve as the expert at court. By issuing a report, that individual takes responsibility for the analysis. This person could be a supervisor or technical leader, but doesn't necessarily need to be the one who did the analysis. The opposition may also call in experts to refute this testimony, and both witnesses are subject to the standard in use by that court (Frye, Daubert, Fed. R. Evid 702) regarding their expertise.

Each court can accept any person as an expert, and there have been instances where individuals who lack proper training and background have been declared experts. When necessary, the opponent can question potential witnesses in an attempt to show that they do not have applicable expertise and are not qualified to testify on the topic. The admissibility decision is left to the judge.


1 293 Fed. 1013 (1923)


Additional Resources


Saferstein, Richard. CRIMINALISTICS: AN INTRODUCTION TO FORENSIC SCIENCE, Pearson Education, Inc., Upper Saddle River, NJ (2007).

McClure, David. Report: Focus Group on Scientific and Forensic Evidence in the Courtroom (online), 2007, (accessed July 19, 2012)


The authors wish to thank the following for their invaluable contributions to this guide:

Robin Whitley, Chief Deputy, Appellate Division, Denver District Attorney s Office, Second Judicial District
Debra Figarelli, DNA Technical Manager, National Forensic Science Technology Center, Inc.

About This Project

This project was developed and designed by the National Forensic Science Technology Center (NFSTC) under a cooperative agreement from the Bureau of Justice Assistance (BJA), award #2009-D1-BX-K028. Neither the U.S. Department of Justice nor any of its components operate, control, are responsible for, or necessarily endorse, the contents herein.

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