Basis & Function of an Expert Witness

The Ohio Rules of evidence state that a qualified expert may give his opinion to help the court understand evidence, or to establish a fact in issue.

Rule 702 controls and is listed below:

RULE 702. Testimony by Experts

A Witness May Testify as an Expert if all of the Following Apply:

  • The witness’ testimony either relates to matters beyond the knowledge or experience possessed by lay persons or dispels a misconception common among lay persons;
  • The witness is qualified as an expert by specialized knowledge, skill, experience, training, or education regarding the subject matter of the testimony;
  • The witness’ testimony is based on reliable scientific, technical, or other specialized information. To the extent that the testimony reports the result of a procedure, test, or experiment, the testimony is reliable only if all of the following apply:
  • The theory upon which the procedure, test, or experiment is based is objectively verifiable or is validly derived from widely accepted knowledge, facts, or principles;
  • The design of the procedure, test, or experiment reliably implements the theory;
  • The particular procedure, test, or experiment was conducted in a way that will
    yield an accurate result.

The Expert Witness Performs Two Primary Functions:

  • The scientific function — collecting, testing, and evaluating evidence and forming an opinion as to that evidence.
  • The forensic function — communicating that opinion and its basis to the judge and jury. A general rule of evidence is that witnesses may only testify to what they have personally observed or encountered through their five senses.

Categories of Expert Witness

An expert may be used in basically two different capacities — consultation or for testimony. Consulting and testimonial witnesses are the basis for expert witnesses. They are derived from five general categories of expertise.

  • Lay people: common sense and life long experience.
  • Technician/examiner: limited and concentrated training, applies known techniques, works in a system and taught with the system [e.g., investigator and supervisors (observers and view- ers)]. The technician is generally taught to use complex instruments (gas chromatographer, infrared spectrophometer, mass spectrophotometer) or even “simple” breath alcohol testing equipment as “bench operators,” who have only a superficial understanding of what the instrument really does, and how the readout is generated. “Bench operators,” who qualify as expert witnesses, are not competent to explain the instrumentation used unless it is established that they received the training and education necessary to impart a thorough understanding of the underlying theories.

In an OVI case, an expert may be called by the defense to challenge the result of the breath test under a reverse extrapolation theory. This theory, in a nutshell, claims that the person who is over the limit at the time of testing passed the .08 threshold subsequent to operation and the arrest.

Further, in a OVI case in which there is a urine result that is over the legal limit, an expert can testify as to a false positive and why that occurs to help a judge or jury understand how the urine is “trapped” in the bladder for a period of time. Thus, if a person did not urinate prior to the test, a false reading may be obtained. Look at it this way: if you have your last of 5 drinks at midnight and go to bed without urinating, the alcohol stays in your bladder overnight. Thus, you may have a urine test result greater than .08 but a breath test of .000. This is known as a false positive.

Qualifications & Competency


The Ohio Rules of Evidence 703-706 cover the requirements and the scope to which an expert may testify.

The witness must be competent in the subject matter. They may be qualified through knowledge, skill, practical experience, training, education, or a combination of these factors. Minimally, the expert witness must know underlying methodology and procedures employed and relied upon as a basis for the opinion.

The background knowledge includes state of art technology, literature review, and experience culminating in an opinion based upon a reasonable degree of scientific certainty. However, there is no absolute rule as to the degree of knowledge required to qualify a witness as an expert in a given field. Once competency is satisfied, a witness’ knowledge of the subject matter affects the weight and credibility of their testimony.