Cross Examining the Expert Witness: Messing with Hypotheticals

Monday, February 11, 2013

I write today about effectively cross examining expert witnesses, which was one of my specialties.

I could write a book about the subject, and maybe someday I will, but today I am going to focus on unraveling the expert’s hypothetical question.

Briefly, Rule 702 of the rules of evidence defines an expert witness as someone who is qualified by education, training or experience to express an opinion regarding a disputed matter in a legal proceeding.

When a jury needs the assistance of an expert witness to decide a disputed matter, either side may present the testimony of a duly qualified expert witness who may express an opinion regarding the disputed matter, including the answer to the ultimate question in the lawsuit.

The party that calls the expert begins by qualifying her as an expert. This normally involves having her tell the jury about her education, training or experience and publications in professional peer reviewed journals. Most experts have a curriculum vitae or CV, that lists their educational credentials and an up-to-date list of their publications. The CV is admitted into evidence.

The expert then tells the jury what evidence she has examined in the case.

Next comes a hypothetical question in which the expert is asked to assume a set of specific facts and asked if she can form an opinion to a reasonable medical or scientific certainty about the significance of those facts.

The witness will answer, “Yes.”

Next question: “What is your opinion?”

Next comes the rat-a-tat-tat of a nail gun.

One of the most effective ways to cross examine an expert is to attack the validity of the assumptions in the hypothetical question.

Undermine one and the expert’s opinion usually falls apart.

Since assumptions typically consist of assuming that disputed facts are undisputed, in a manner that benefits the party that called the expert, the opposing party cross examining the expert simply asks the expert to assume the contrary view.

Another way to challenge an expert opinion is to introduce an additional assumption into the hypothetical that undermines the conclusion.

Let us take, for example, the issue regarding whether Trayvon or the defendant was on top when the defendant fired the fatal shot.

The undisputed facts are the trajectory of the shot, which was direct from front to back, the intermediate range from which the shot was fired, the nonalignment of the two aligned holes in the sweatshirts with the entry wound and the muzzle of the gun was in contact with the outer sweatshirt when the fatal shot was fired.

The disputed fact is whether Trayvon or the defendant was in the superior position straddling the other.

The defense probably could find an expert who, given those undisputed facts, could testify to a reasonable scientific certainty that the defendant was on the bottom when he fired the fatal shot.

The problem with the hypothetical, however, is that it does not account for the presence of the gun in the defendant’s hand.

According to the defendant’s statement, he pulled the gun out of the holster that he carried inside the waistband of his jeans behind his right hip.

The expert’s opinion falls apart when you add this undisputed fact to the hypothetical because the defendant could not draw the gun and get it into the necessary position to fire the fatal shot.

Note BTW, that one also could conclude to a reasonable scientific certainty from the original set of undisputed facts that the defendant was in the superior position straddling Trayvon.

This is how you shoot down credentialed experts in just a few words.