Elements of Expert Testimony

By Pat A. Wertheim

This article originally appeared in Minutiae, the Lightning Powder Co. Newsletter, #44, Sep-Oct 1997

When offering testimony as an expert witness, regardless of the witness’ discipline, five distinct topic areas must be covered. These five areas are the witness’ qualifications, the science practiced by the witness, the introduction and chain of custody of the evidence, the analysis or examination process, and the expert’s opinion. By adequately covering each area in sequence, the witness will five the judge or jury all of the information necessary to understand the opinion presented.

The first area of testimony should always be the witness’ qualifications. There are three ways to prove you are qualified. The first is by explaining the training you have had. Include formal education, such as college, as well as specialized classes in the science completed. The second way of proving your qualifications is through experience. Talk about how many years you have been doing the job, or how many cases you have examined. The third way of proving your expertise is through explaining how you have been tested and found competent. There are several ways you may have been tested. A test for certification in your field is one. Annual proficiency testing by your lab is another. Previous testimony itself is a way of showing that you have been found competent by the courts.

After testifying to your qualifications, you should next explain your science. This segment of testimony actually serves two purposes. First, you familiarize the judge or jury with the basics they need to understand in order to appreciate you opinion. Second, by assuming the role of teacher, you reinforce your status as expert as set forth n the first segment of testimony. In testifying about your science, you may want to place special emphasis on the particular techniques in this case to set the stage for your testimony in the fourth area, the analysis or examination process. For example, if you are testifying to a latent print developed with Ninhydrin on a forged check, your discussion in area two would concentrate on chemical development on porous surfaces rather than an explanation of use of powders on nonporous surfaces.

The third area of testimony is the introduction of the evidence and establishment of correct chain of custody. In this regard, it is important to be certain that the wording chosen is literally correct and reflects only the extent to which you were personally involved. For example, in testifying to a latent print lift from the rearview mirror of the recovered stolen car, you would testify to “the fingerprint found on the rearview mirror” only if you actually found it there yourself. If someone else developed the latent print and submitted the lift to you for comparison, you would testify to “the latent print lift labeled ‘rearview mirror'”.

Once you have introduced the evidence, you would testify to the specific examination process in this case. Here, if you have laid the foundation well in testimony area two, the judge or jury will fell a sense f familiarity with your explanation.

Finally, you should testify to the conclusion you have reached as a result of your analysis or examination. If you have first established that you are an expert in your field, explained the science adequately, introduced the evidence correctly, and described your examination of the evidence, your conclusion will be understood and accepted by the judge or jury.

It is important to recognize that an expert witness should not be biased in the case. You should not stretch the meaning of the evidence nor overstate the meaning of the evidence. It is the attorney’s responsibility to prove the case, not yours. The judge or jury should therefore understand the limitations of the evidence as well as the possible meaning.

Ideally, the calling attorney should follow a logical sequence in asking his questions. In reality, however, he or she may be unfamiliar with your science or with introducing expert testimony. If you cannot have a pretrial conference with the attorney, it helps to go to court with a suggested list of questions. Most attorneys are grateful for the help and will use your questions.

For more information on preparing such a list of questions, read the article, “Qualifying as an Expert Fingerprint Witness: Designing a Set of Questions to Assist in Court Testimony” published in the Journal of Forensic Identification, March/April, 1990.

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