Expert evidence is offered during a trial by putting an expert in a given field on the stand to testify as to his or her opinion on a matter in the case. In order for a judge to allow one of the parties to present expert evidence to the jury through testimony, there are typically four qualifications that must be met. The subject matter must be appropriate for expert evidence, the witness must be qualified to give his or her expert opinion on the matter, the expert must assert a reasonable probability that his opinion is conclusive on the matter, and the opinion must be supported by a stated factual basis.
The first qualification to the admission of expert evidence in a trial is that the subject matter be appropriate for such evidence. The subject matter of the stated opinion must be one where scientific, technical, or other specialized knowledge would assist in coming to a conclusion on the matter. For instance, an expert on brakes would be able to testify as to how fast a car was likely going based on the skid marks it left. Conversely, expert testimony is not necessary to estimate how fast a car was going if the witness actually saw the moving car.
Any witness whose opinion is offered as expert evidence must be qualified to give such evidence. In other words, he or she must have knowledge, skill, experience, or education that indicates that he or she is an expert on the matter. The basis for such a qualification must be explained to the jury in order for the opinion to be deemed relevant.
The expert must express a reasonable amount of certainty that his or her opinion is conclusive on the matter. Courts do not typically allow mere guesses to be offered as expert opinion testimony, even if it could be characterized as an “educated” guess. For example, a medical expert may not testify that certain symptoms “may in some cases” indicate a disease. Rather, he or she would have to testify that in his or her opinion it is reasonably certain the subject had the disease.
The final qualification to allowing expert evidence in a trial is the factual basis on which it is stated. The basis for an expert’s opinion must be on his or her own personal experience, evidence submitted at trial and presented to the expert, or facts of a type that are typically relied upon by others in the expert’s field. Very often, a party will present research papers or books containing technical knowledge, referred to as “learned treatises,” as the basis for expert evidence.