Admissibility of Expert Testimony
By Aaron Larson
March, 2005
Contents
What Is An Expert Witness
An expert witness is a witness who, by virtue of specialized knowledge, skill, education, training, and experience, is deemed competent to provide opinion testimony in relation to the matter before the court. Not all experts will be formally trained. For example, the owner and manager of a shooting range may be qualified as an expert in relation to certain aspects of firearms, or the proper operation of a shooting range, by virtue of experience alone.
What Is Opinion Evidence
Traditionally, a non-expert or "lay" witness is expected to testify to the facts of a case, and allow the jury to determine what the facts mean. Opinion testimony interprets and explains the facts. For example, fact testimony about a car accident might include testimony about the skidmarks behind the plaintiff's vehicle, but a qualified expert can analyze the skidmarks and testify as to an estimate of the defendant's speed.
Experts may also answer hypothetical questions, so lawyers may use an expert to try to advance a particular theory of a case or to undermine competing theories. For example, if a defendant were to claim that he did not cross the center line of a road ot cause an accident, the expert might testify as to the location of damage on the vehicles involved in the accident, as well as the location of skid marks and debris on the roadway after the accident, and present first an opinion as to how the accident occurred, and second an opinion that the defendant's theory was unlikely based upon the evidence at the accident scene.
Non-expert witnesses can provide opinion testimony within their sphere of knowlege and experience. For example, a witness might be permitted to provide testimony that he believed the defendant to have been drunk at the time of the accident, based upon such factors as the defendant's appearance, behavior, and odor, because most people have sufficient experience with alcohol and alcohol intoxication to form an opinion as to whether or not a person is drunk.
Opinion evidence is not supposed to take the place of the jury's independent determination, but is intended to help the jury understand the facts and issues presented to it. Presenting the jury with the results of a DNA test, for example, is of little utility unless an expert explains and interprets those results for the jury. However the expert is ordinarily forbidden to testify as to the "ultimate issue" before the jury - that is, an expert might opine that it
Requirements To Introduce Opinion Evidence
Historically it was often necessary to detail an expert's background to a court, then to formally request that the court recognize the witness as an expert, before the witness would be allowed to provide expert testimony. That formalistic requirement has largely been eliminated, such that a witness does not need to be formally designated as an expert witness by a court before providing opinion testimony. However, the parties will still typically introduce evidence of their experts' qualifications in order to try to buttress the expert's credibility with the jury. Also, the opposing party may object to the opinion testimony on the basis of the expert's lack of qualification, in which case the trial judge will nonetheless have to rule on the expert's qualification.
Prior to the introduction of opinion testimony, the proponent of that testimony (the party seeking to have it admitted) will have to lay a proper foundation for its admissibility. If the party cannot connect the desired expert testimony with the facts of the case, the judge may exclude the testimony. If the testimony is scientific in nature, part of laying a proper foundation is to establish the validity of the science and scientific method used by the expert to form a particular opinion. Often this determination will be made in advance of trial, so that the parties can anticipate the expert trial that will be admitted at trial, and so as not to delay the trial by holding hearings on the admissibility of the expert testimony after the trial has started.