How Do I Challenge an Expert Witness?

Often in criminal cases Expert Witnesses are called to offer expert testimony at trial. A witness who is qualified as an expert may offer an opinion on a topic. For example, the expert may say that in his opinion the bullet recovered from the body was fired by this gun or the note handed to the bank teller was written by the Defendant. (Lay witnesses are not permitted to offer opinion testimony.) Yet, far too often the expert testifies without challenge from the defense. More troubling, sometimes the "expert" is not really a qualified expert or the method he used to reach his conclusion is faulty.

If you are confronted with an expert who will testify against you, your attorney should first consider filing a Daubert motion. A Daubert motion is a request for a judicial hearing for the purpose of challenging or questioning an anticipated expert witness and his or her intended testimony. It is called a Daubert motion due to the United States Supreme Court case which established the guidelines for determining the admittance of expert testimony. (Daubert v. Merrell Dow Pharmaceuticals, Inc. (1993), 509 U.S. 579.) In Daubert and Kuhmo Tire Company v Carmichael (1999), 526 U.S. 137 the Court established criteria for judges to follow in determining whether or not an witness should be permitted to testify as an expert.

At a Daubert hearing (conducted before trial) two primary questions are addressed. The first area of concern is the expert herself. Is the intended witness actually an expert, does she have the education, training and experience to testify as an expert? Second, the method or process used by the intended witness will be in question. The primary focus will be on the reliability and validity of the method or process. Reliability is the degree to which an assessment tool produces stable and consistent results on repeated trials or tests. Validity is the extent to which the method actually measures what it says it will measure. For example, suppose I claim my newly devised BS 1000 can test blood sugar levels using harmless radio waves, thereby eliminating the need for blood draws or finger pokes. If my new machine takes the blood sugar levels of 100 people and accurately measures the level of 97, we would say it was reliable. However, if there is no scientific explanation as to how radio waves can measure blood sugar, then the BS 1000 would not be deemed valid.

If successful, a Daubert challenge, may be the key to your defense as the expert will not be allowed to testify against you. (In another blog I provide some insights into preparing for a Daubert hearing) If your challenge not successful, you should at least have the information necessary to effectively cross-examine the expert at trial.