Evidence Law: Opinion Testimony of Laypeople and Experts
During a judicial hearing or trial, many different types of testimony will be offered. One area of testimony that is given special treatment is opinion testimony.
The first principle of opinion testimony is that it is disfavored. The general preference is that witnesses tell the judge or jury what they know and let the triers of fact draw conclusions that are warranted. There is therefore a general principle that testimony as to the witness’ opinions will not be allowed unless there is a good policy reason to allow it.
Still, opinion testimony can be necessary, helpful and admissible under a variety of circumstances. We will focus on two types of opinions. The first is that given by an ordinary or “lay” witness. The second is opinion testimony coming from someone who is qualified as an “expert” in the relevant field. Because the rules vary greatly between how these two types of opinions are treated, we will deal with them separately.
Lay Witness Testimony
Opinion testimony offered by a lay witness is limited in three important ways. To be admissible, lay witness opinion testimony must:
· be rationally based on the witness’s perception;
· be helpful to clearly understanding the witness’s testimony or to determining a fact in issue; and
· not be based on scientific, technical or other specialized knowledge
Lay witness testimony often begins with the witness testifying to observations that he or she personally perceived. For example, say that a witness states: “I observed a black truck approach a red light and continue driving through it without stopping.” This is testimony as to a factual observation. Next, the witness states, “The black truck appeared to be moving faster than the posted 30 miles per hour speed limit.” This statement is an opinion, since the witness has no reliable scientific basis upon which to be sure that this is accurate.
Still, the latter statement would likely be accepted by the court because it satisfies the three elements discussed above; it is based on what the witness perceived, is helpful to determining how the collision occurred, and is not based on scientific, technical or specialized knowledge. The witness was careful to provide only an approximation of speed because providing the precise speed would be beyond what anyone could reasonably determine from simply observing a moving vehicle. Moreover, an ordinary person would be expected to be able to approximate the speed of a moving vehicle. Finally, it is essential to allow this testimony because there is no other reasonable way to convey this important idea to the jury other than through opinion testimony.
On the other hand, if the witness stated “the car appeared to be driven by somebody who was intoxicated,” this may not be allowed. While the opinion may be reasonable, based on observations, it is better for the witness to simply testify as to her direct observations. She might, for example, state that the car was swerving or moving in and out of lanes at random. The trier of fact can then decide whether those observations warrant a conclusion that the driver was intoxicated.
Expert Witness Testimony
Experts are given more latitude in expressing opinions than are lay witnesses.
To qualify as an expert witness, the witness offering the testimony must meet preliminary requirements. First, prior to offering an expert opinion, a witness must be formally recognized by the court as an expert in an area of expertise that relates to the opinion. This requires that the witness testify to having knowledge, skill, experience, training, or education in the area of expertise. For example, in a car accident case, one party may wish to present a medical opinion regarding his back injuries. A witness may be qualified as an expert by demonstrating (usually through his own testimony) that he is a medical doctor currently licensed and is certified by the American Board of Physical Medicine and Rehabilitation in spinal cord injuries and has years of experience in the field, has performed relevant procedures and has published in medical journals. Based on this testimony, the court can accept the witness as a medical expert. Of course, if challenged, the court may require that the credentials be verified before accepting a witness as an expert.
Once a witness is accepted by the court as an expert witness, the expert may offer an opinion if:
· the expert’s scientific, technical or other specialized knowledge will help the court or jury understand the evidence or determine a fact in issue;
· the opinion is based on sufficient facts or data;
· the opinion is the product of reliable principles and methods; and
· the expert has reliably applied the reliable principles and methods to the facts.
Not only does the witness need to have a background that qualifies him in the area, but he must also apply the knowledge he has in a manner that the court finds reliable. This involves focusing on the theories and techniques underlying an expert’s opinion. The factors a court may consider in assessing an expert opinion include:
- whether the underlying theory or technique has been tested and subjected to peer review,
- the known error rate for the theory or technique, and
- whether the theory or technique is generally accepted by experts in the relevant scientific community.
Expert opinions are deemed reliable if they stem from principles and methods that have been tried and tested over several years. Novel theories and techniques pose more of a problem simply because there is less research and data supporting them.
Expert Witness Opinions
There are two important distinctions that differentiate opinions offered by lay witnesses and expert witnesses.
First, expert witnesses do not need to personally perceive or observe the facts that form the basis of their opinions. Frequently, expert opinions are formed from a combination of facts that they have learned from the case, such as the details of a car collision, and data they know from their field, such as medical principles and studies. If the expert relies on facts or data that an expert in that field would reasonably rely on, then those facts and data do not need to be introduced into evidence. Thus, if the medical expert bases his opinion about the party’s spinal injury on a medical study he reviewed in a medical journal, the article does not have to be admitted into evidence with his opinion.
If the expert relied on information that is not ordinarily admissible, then that information may only be admitted if its probative value outweighs any prejudicial effect. In other words, the inadmissible piece of information must be so valuable to the case that its value outweighs any harm the information may cause to a party.
So, for example, if a medical expert seeks to testify to an opinion that is based partially on hearsay that would not be admissible (perhaps something he heard while conducting an interview with a witness to the incident), the other party may be able to get the hearsay admitted to support an argument that the expert misrepresented or misunderstood something the witness said. Commonly, this issue arises during cross-examination of an expert witness where the person conducting the cross-examination asks the expert to disclose the facts or data he relied on in forming his opinion. While the expert was not required to disclose this information in the first instance, if he is asked to provide this information on cross-examination, he must do so. 
Second, unlike expert witness opinions, lay witness opinions are limited to matters that are within the scope of everyday, commonplace experiences and understanding. A lay opinion must be the sort of thing that an ordinary person would infer under ordinary circumstances. An expert opinion has no such limitation.
The historical rule was that witnesses, whether laypeople or experts, were not supposed to pass judgment on the ultimate issue in controversy. That was the job of the finder of fact. While a witness may testify as to how fast a car was moving, the theory went that the witness should not testify that she believes the driver of the red car was at fault in the accident. Determining who was at fault is the jury’s job.
The Federal Rules of Evidence have abandoned this historical rule as Rule 705 states that testimony is not objectionable just because it “embraces an ultimate issue.” Of course, opinions as to the ultimate issue in controversy must meet the other rules that are applicable to opinion testimony. Thus, opinion as to who’s fault an accident is may be inadmissible simply because it’s not warranted by the observations or not necessary to assist the trier of fact.
However, with regard to criminal trials, the Federal Rules do provide that a “witness must not state an opinion about whether the defendant did or did not have a mental state or condition that constitutes an element of the crime charged or of a defense.” For example, in a murder trial, a witness may say “John pointed the gun, informed the victim that he would shoot and did so,” but may not say “John intended to kill the victim.”
In all cases, opinions must be supported by adequate foundations. Both lay witnesses and expert witnesses must offer testimony establishing that the witness is the appropriate party to offer an opinion and that the opinion stems from an appropriate basis. The court has the role of making the final decision on these matters and has discretion to exclude opinions that are not reliable or come from witnesses who are not qualified to offer them. When opinions are properly presented to the court, they can be powerful tools in determining facts and issues in a case.
 Fed. R. of Evid. 701.
 Fed. R. of Evid. 702.
 Daubert v. Merrell Dow Pharms., 509 U.S. 579 (1993).
 Fed. R. of Evid. 703.
 Fed. R. of Evid. 705.
 Fed. R. of Evid. 704.