Expert witnesses are increasingly facing Daubert and other challenges to their expert witness testimony and opinions. What can expert witnesses to do to increase the likelihood that their testimony will survive such challenges? Attorney Adam Bain, writing for SEAK, provides four tips to prevent exclusion of your expert witness opinions.

(1) Ensure that you have enough information to reach your opinions.

Lawyers are often selective in the information that they provide to experts. A lawyer may not provide an expert with all of the information that the expert needs to render expert opinions. There are several reasons why this may occur. The lawyer may be inexperienced, may be trying to control costs, or may be attempting to influence the expert’s opinions.

At the time you agree to assist in a case you should ensure that you will not be denied access to any information that is necessary for you to reach your opinions. A good opposing lawyer will spend time in discovery focusing on the extent of the information that you considered in forming your opinions. The lawyer will focus on whether you were provided with all of the information that the lawyer believes is necessary for you to render your opinions. The lawyer will want to know whether you considered all of the documents, data collections, deposition testimony, opinions of other experts and other information that would be necessary for you to form your opinions.

Because Federal Rule of Evidence 702(a) requires that expert opinions be based on sufficient fact or data to be admissible, a failure to consider certain facts or data can be a basis to exclude your opinion.

(2) Ensure that you follow the core principles and methods of your field in reaching your opinions.

Certain fields have developed core principles and methods. For example, to arrive at a diagnosis of disease, a physician will consider all possible diseases that could produce the observed signs and symptoms in a patient and determine which of the several possible diseases is most likely responsible for those signs and symptoms. This “differential diagnosis,” is a standard methodology for diagnosing illness. Thus, a physician witness who proposes to testify regarding disease should ensure that a differential diagnosis underlies the identification of the disease. There are other examples of core principles in different fields. A statistician or an epidemiologist will use an appropriate control group before reaching opinions on causation. An economist will consider the appropriate interest and inflation rates to reduce a future income stream to present value.

Thus, when you are formulating your opinions you should make sure that you are not ignoring or violating the core principles and methods of your field. To best determine the core principles and methods in your discipline you should consult the authoritative resources in your field. The Federal Rules of Evidence allow lawyers to confront experts with statements in authoritative treatises to test the validity of their opinions. This does not mean you need to reach the same conclusions that are within authoritative works to testify in court. But, if you fail to follow standard principles and methods, lawyers will likely challenge the reliability of your opinions and seek to exclude them. An important Daubert factor is the “existence and maintenance of standards controlling” the operation of a theory or technique. If you violate a standard in your field, a lawyer may cite this Daubert factor to exclude your opinions.

In addition to the authoritative resources in your field that you should consult to ensure that your opinions can survive a Daubert challenge, there is another important resource to consider, the Reference Manual on Scientific Evidence published by the Federal Judicial Center. Because Daubert and its progeny have required judges to become more adept at analyzing expert opinions, the Federal Judicial Center published this resource to assist judges in evaluating expert witness testimony. The Reference Manual, now in its third edition, discusses the Daubert trilogy, as well as the methods and procedures of science. Significantly, it also contains several “reference guides” on several expert disciplines; including statistics, economic damages, toxicology, and engineering (more are added with each edition). If you are testifying to one of the areas covered by a reference guide in the Reference Manual, you should review that reference guide to make sure that your work is consistent with the guide. The Reference Manual itself is publicly available on the internet.79 Lawyers and courts often cite the Reference Manual in challenging and evaluating the reliability of expert testimony.

(3) Do not create a novel methodology to arrive at your opinions.

Perhaps the greatest risk of exclusion of expert testimony occurs when an expert creates a novel methodology specifically for the purposes of litigation. There can be a great temptation to do this when the standard methodologies do not provide an answer to a proposed question. Or, an established methodology may provide an answer, but it is not an answer that helps the case. Experts can get into trouble when they seek to push an established methodology beyond its intended purpose or create a new methodology using bits and pieces of established methods.

Invariably, when confronted, experts will claim that they are following standard methodologies to reach their conclusions. But experts should assume that there will be a highly qualified expert working for the opposing attorney to scrutinize closely the expert’s proposed testimony to determine exactly what the expert did to reach the proposed opinions. In federal court, experts must provide a detailed expert report before testifying which gives the opposing party ample opportunity to conduct such an inquiry.

If it is shown that you have created a new methodology for purposes of litigation, it will be difficult for you to prove that expert opinions based on that methodology are sufficiently reliable to be admissible. First, it will be difficult to establish the reliability of a new methodology through application of the Daubert factors. It is unlikely that a new methodology that you created has been subject to peer review or publication or has a known rate of error. Further, a new methodology, because it is new, has not gained general acceptance in the relevant scientific community. Finally, as recognized in the Notes to Rule 702, courts are skeptical of new methodologies created for purposes of litigation that do not have application outside of the courtroom. Thus, when an expert has “developed . . . opinions expressly for the purposes of testifying” or is not using the same degree of care as in “regular professional work” the expert runs a great risk that a court will find the testimony unreliable and exclude it.

(4) Have a detailed and well-founded explanation of how and why an application of the methodology to the facts led to your expert opinions.

Experts sometimes overlook the need to provide a detailed explanation of how and why application of a reliable methodology to the facts of a case led to their expert opinions. Because Rule 702 requires courts to ensure that an expert has “reliably applied the principles and methods to the facts of the case” before admitting expert opinion testimony, the ability to explain the application of a methodology is critical.

For example, an expert who relies on experience to reach an opinion may believe it is sufficient to state simply “I used my experience.” But the Notes to Rule 702 show that even an experience-based expert must explain how experience reliably led to the expert opinion.83 The Notes to Rule 702 also provide two considerations for courts in scrutinizing an expert’s application of a methodology to facts of the case. First, an expert’s application of a methodology will be unreliable if it “unjustifiably extrapolated from an accepted premise to an unfounded conclusion.”84 Second, an expert’s application of a methodology will be unreliable if it has not “adequately accounted for obvious alternative explanations.”85 Thus, in explaining the application of your methodology, you should be able articulate in detail: (1) why any extrapolation from a premise to a conclusion is justified, and (2) how you accounted for alternative explanations to the opinions you reached.

Your explanation of how and why the application of a methodology to the facts of the case led to your expert opinions should be an explanation that an educated layperson can understand. The explanation should make sense and be persuasive. This will not only help ensure the admissibility of your expert opinions but will make it more likely that the factfinder will accept your opinions as the more likely explanation for what occurred.


Given the consequences of a court finding that your expert testimony should be excluded as unreliable, the value of knowing the standards for admission of expert testimony is apparent. If you know the standards you can guard against offering opinions that are vulnerable to exclusion. Additionally, ensuring that your opinions meet the standards of evidentiary reliability will also help guarantee that your opinions will assist the finder of fact in reaching a just resolution of the dispute, which is the underlying purpose of the admissibility standards and the ultimate goal of our judicial system.

Adam Bain, Esq. has been an attorney in the United States Department of Justice for over 25 years. During that time, his primary responsibility has been defending the United States in toxic tort litigation. In his litigation, Mr. Bain has worked with some of the nation’s top experts in toxicology, epidemiology, psychology, hydrogeology, real estate appraisal, and environmental sciences. He is a frequent faculty member at the Department of Justice’s National Advocacy Center where he often lectures on finding and using expert witnesses. Mr. Bain has also spoken at the SEAK National Expert Witness Conference. He often writes on expert witness issues. Mr. Bain obtained is B.A. (1983) from Purdue University and his J.D (1986) from Indiana University Maurer School of Law.