Expert Witnesses – Demonstrative Evidence

From Qualifying & Attacking Expert Witnesses by Robert Clifford

It is common knowledge that we live in a visual society: people comprehend and retain what they see rather than simply what they hear. Consequently, demonstrative evidence plays a critical part in conveying an expert’s opinion. However, counsel must consider carefully the demonstrative evidence that is to be presented. Make certain that charts are large enough to be seen by all jurors, that the material is not too complicated or confusing, that diagrams clearly depict what the expert says they depict, that graphs are accurate, and that the material is displayed long enough for the jury to comprehend what is being shown.

Experts frequently testify about the operation of machinery which involves comprehension by the jury of the spatial relationship and the movements of various parts. If the workings of a complicated mechanism are significant, it is simpler for the jury to comprehend how the machine operates by watching a model operate rather than relying upon an expert’s oral description. Without a model, an expert’s explanation of how a machine operates can be time- consuming and still not be fully understood. In automobile case, the bank or grade of a roadway may be significant and a three-dimensional model showing vertical as well as horizontal dimensions, indicating lines of sights and obstruction, may be vital to an accident reconstruction expert’s testimony.

In addition to using demonstrative evidence as an adjunct to an expert’s testimony to illustrate a particular point, a model may be used by an expert to prove an element of the case. For example, a model used in a water-contamination case to establish that the defendant’s conduct caused the contamination of the water supply of a nearby area may be proved by using a model based upon the same information such as water flow, topography, and porosity.

Bob Clifford, author of Qualifying and Attacking Expert Witnesses, resides in Carmel, California. He serves as a consultant to law firms and as an arbitrator and mediator in insurance and litigation matters. He has been the senior partner of an Oakland, California law firm where he specialized in general litigation, including real property disputes, personal injury litigation, insurance matters, contract disputes, will contests and estate matters.


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