EXPERT WITNESSES

As stated before, it is your responsibility as jurors to determine how credible each witness is, including the experts. In determining the credibility of any expert testimony, you may wish to determine how reliable the content of the expert’s testimony is. Reliability refers to how good the methods used by the expert in making conclusions are. The U.S. Supreme Court suggested some things for judges to consider when they are determining how reliable an expert is, and these may also be useful for you as jurors in determining how reliable an expert is. You may consider all, some, or none of these things in determining credibility. They include:

1) Falsifiability: This refers to whether or not the theories the expert talks about can be tested. In other words, is it possible to create a situation or a study that would be able to prove the theory to be true or false? One of the qualities that makes a something scientific is that it can be tested or "falsified." A theory or hypothesis is falsifiable if we can study or test it to prove that it is true. For example, imagine a case where a tire on a car explodes and the passengers in the car are injured. An expert in tires testifies that he can tell if the tire explosion was caused by the driver of the car or the people who made the tire. He has a theory that the tread tears off of the tire differently if the driver is at fault versus the tire manufacturer. Something you may want to ask yourselves as jurors is whether or not his theory can be or has been tested. Has he been able to or would he be able to test some tires which have been damaged to prove his theory about the tread tearing is true? In our example, if the tire expert can perform a test on the tire which tells him the cause of the explosion, then it is falsifiable. If there is no way to set up a test of his theory, and it is just a theory, then his theory is not falsifiable.

2) Known or Potential Error Rate: This refers to how often the expert is wrong when he is applying his theory or methods. In other words, when an expert is saying that his hypothesis is true, what is the chance that he will be wrong? If the error rate for a method is very low, not wrong very often, then it would be more reliable than if the error rate was very high, or wrong a lot of the time. For example, the tire expert may have a specific test he conducts on faulty tires. He may be trying to determine if the tire failed because of a manufacturing defect. The error rate of this test would be how often the expert said the tire failed because of the driver when in reality the tire failed because of the manufacturer. In other words, the error rate of the test would be how often the expert was wrong. Something you may want to ask yourselves about the expert in this case is do we know how often his test results are wrong? If we do know, then how often are they wrong and does this seem like they are wrong too often or not? In our example, if the tire expert could perform tests on many tires, some where he knows the cause is driving and some where he knows the cause is in how the tire is made, then he would know what his error rate was, or how often his test is wrong.

3) Peer Review or Publication: This refers to whether or not the topic of the expert’s testimony has been reviewed by other experts or has been published. One of the ways experts judge the work of other experts in their field is to see how many times that expert has had his or her work published. There are different types of publications. One type of publication is a "peer reviewed" journal. This means that before an article is published, it is reviewed by other accomplished experts in the field. These other experts make a recommendation on whether the article should be published or not. This is the highest quality type of publication because peer review rigorously evaluates the reliability of the research methods in the article. Other types of publications are not peer reviewed, but have gone through some review process before they were published. More publications, especially in peer reviewed journals, typically means that the expert has more experience and that his or her research has been positively evaluated by other people in the area. Therefore, you may want to take into account not only where the expert has been published but also how many times he or she has been published. In our example, the tire expert may have published his theory about tire testing, or he may not have. A question you may want to ask yourself is how many times has the expert’s work been published and what kind of publication was it?

4) General Acceptance: This refers to whether or not other people who are experts in the same field as the expert who testified in the trial agree with that expert’s methods and/or conclusions. A theory, methodology, or conclusion is generally accepted if many other experts who work in the same area as the expert who has testified before you would agree with that expert. It is thought that the more people who agree on a theory or a method, the better or more reliable that theory or method is. For example, the tire expert testifies about a test that can be done on the damaged tire to determine if the tire explosion was caused by the driver of the car or the people who made the tire. A question you might ask yourselves as jurors is whether or not other experts in the field of tire testing would agree that the test performed by the expert was a reliable one. In our example, if other tire experts would agree with what the tire expert in the case said, then that testimony would be "generally accepted" by tire experts.