Generally speaking, the law of evidence in both civil and criminal cases confines the testimony of witnesses to statements of concrete facts within their own observation, knowledge, and recollection. Testimony must normally state facts perceived by the witnesses' use of their own senses, as distinguished from their opinions, inferences, impressions, and conclusions drawn from the facts. Opinion testimony that is based on facts is usually considered incompetent and inadmissible, if the factfinders are as well qualified as the witness to draw conclusions from the facts.
In certain instances, however, the law allows witnesses to provide opinion evidence, and such evidence is divided into two classes, lay opinion and expert opinion. A lay witness may give his or her opinion when that opinion is (1) rationally based on the perception of the witness; (2) helpful to a clear understanding of the witness' testimony or the determination of a fact in issue; and (3) not based on scientific, technical, or other specialized knowledge within the scope of expert testimony discussed below. Thus, lay witnesses who have had an opportunity to observe a particular vehicle in motion are normally permitted to testify that it was traveling ata great rate of speed or was going pretty fast. Lay witnesses are also normally allowed to give their opinion as to the height, weight, quantity, and dimensions of things, even if their testimony is not precise. By definition, a lay witness is any witness who is not qualified to testify as an expert on a particular subject.
Expert witnesses are persons who are qualified, either by actual experience or by careful study, to form definite opinions with respect to a division of science, a branch of art, or a department of trade. The law deems persons having no such experience or training to be incapable of forming accurate opinions or drawing correct conclusions. Thus, if scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify in the form of an opinion or otherwise, if (1) the testimony is based upon sufficient facts or data; (2) the testimony is the product of reliable principles and methods; and (3) the witness has applied the principles and methods reliably to the facts of the case. In Kumho Tire Co. v. Carmichael, 526 U.S. 137, 149-152, 119 S.Ct. 1167, 143 L.Ed.2d 238 (1999), the U.S. Supreme Court further observed that the reliability of a scientific technique may turn on whether the technique can be and has been tested; whether it has been subjected to peer review and publication; and whether there is a high rate of error or standards controlling its operation.
Courts do not apply a rigid rule in determining whether a particular witness is qualified to testify as an expert. Instead, an expert's qualifications are normally evaluated on a witness-by-witness basis, according to the facts and issues of each case. Several courts have stated that the true criterion in determining the qualification of expert witnesses is not whether they employ their knowledge and skill professionally or commercially, but whether the jury can receive appreciable help from them on the particular subject in issue. Many courts also require the witness to exhibit sufficient knowledge of the subject matter before his or her opinion to go to the jury.
The qualifications of an expert witness must be carefully scrutinized by courts to guard against charlatans who may give erroneous testimony without a sound foundation. Most courts will more closely scrutinize the qualifications of witnesses seeking to testify as experts if they have never been found qualified to give expert testimony on a prior occasion. However, primary reliance is not placed on the fact that it may be the expert's first time on the witness stand. Conversely, the fact that a witness has been previously qualified to give expert testimony on the subject matter in question is typically irrelevant to his or her qualifications for giving such testimony in a subsequent case.
There are two general classes of matters as to which expert testimony is admissible: (1) matters as to which the conclusions to be drawn by the jury depend on the existence of facts that are not common knowledge and that are specifically within the knowledge of persons whose experience or study enables them to testify with authority on the subjects in question; and (2) matters as to which the conclusions to be drawn from the facts stated, as well as knowledge of the facts themselves, depend on professional or scientific knowledge not within the range of ordinary training or intelligence. In the first class, the facts are stated by the experts, and the conclusion is drawn by the jury. In the second class, the expert sets forth the facts and states a conclusion in the form of an opinion which may be accepted or rejected by the jury.
Accident reconstruction experts typically give testimony that falls into the first class of expert testimony. Such experts may testify as to the speed at vehicles were traveling, the distance before impact at which each driver began applying the breaks, and what, if any, accident-avoidance precautions each driver took. But accident reconstruction experts are not allowed to give their opinion as to which driver was responsible for the accident or testify as to the standard of care required to be exercised by the drivers. Both types of questions are ultimate issues that only a jury can determine. By contrast, in medical malpractice cases physicians may provide the jury with testimony regarding the underlying facts of the legal dispute and may aid the jury by describing the standard of care for diagnosis and treatment.
The general rule excluding opinion evidence concerning matters of common knowledge or experience, while clear as a matter of principle, is frequently difficult to apply. As a result, courts are given wide latitude in determining whether the opinions of an expert or lay witness are admissible, and appellate courts will not interfere with a lower court's ruling unless in making that ruling the trial court manifestly abused its discretion to the prejudice of the complaining party.