Five Essential Considerations for Expert Witnesses


Five Essential Considerations for Expert Witnesses



Hon. Robert Trentacosta


Expert witnesses are seemingly ubiquitous in litigation. Cases that involve scientific, medical, technical or specialized evidence will almost always require experts to explain the evidence to lay jurors. Once you decide that you need an expert witness to prove your case, the following are five considerations that are essential to the use of your expert witness.

I. Know the Essential Qualities of an Effective Expert:

  • Clarity of testimony. The best experts are effective teachers. They serve as translators for jurors by explaining complex matters in a simplified, understandable, and persuasive way. If the expert cannot make his or her testimony understandable to jurors, the expert is not useful.
  • Qualifications (Credentials). An expert must have sufficient qualifications that would allow him/her to render credible opinions and conclusions. Experts with impressive degrees, training, and experience have weight and power with jurors. The witness must be an expert in the subject matter of their testimony.
  • Actual Work Performed. The expert must do the work and perform a methodical and logical analysis of the contested issue that will overcome the rigors of cross-examination.
  • Frame Logical, Fact-Based Conclusions. The ultimate opinions and conclusions of the expert must logically and ineluctably follow from their expertise and work product. They must be able to defend their opinions and conclusions with unshakeable confidence.

II. Showcase Your Expert’s Qualifications on Direct Examination:

Evidence Code §720(a) allows a person to testify as an expert witness “if he has special knowledge, skill, experience, training, or education sufficient to qualify him as an expert on the subject to which his testimony relates.” (emphasis added).

Evidence Code §720(b) allows the proponent to show the expert’s necessary special knowledge, skill, experience, training or education “by any otherwise admissible evidence, including his own testimony.”

Your initial goal on direct examination will be to demonstrate that your witness has the necessary qualifications listed in Evidence Code § 720(a). By showcasing the expert’s qualifications, you will enhance his or her credibility with the jurors. Ask open-ended questions and allow the witness to describe his/her qualifications in detail. For example:

“Please describe your training in the area of….”

“What specialized education do you have in the field of…”

Evidence Code §767(a) states that “Except under special circumstances where the interests of justice otherwise require:

  • A leading question may not be asked of a witness on direct or redirect examination.” (emphasis added).

Despite this seeming prohibition, the Comment to Evidence Code § 767 states:

“The exception stated at the beginning of the section continues the present law that permits leading questions on direct examination where there is little danger of improper suggestion or where such questions are necessary to obtain relevant evidence. This would permit leading questions on direct examination for preliminary matters, refreshing recollection, and examining handicapped witnesses, expert witnesses, and hostile witnesses” (emphasis added).

While most courts grant counsel some latitude in asking leading questions of an expert on direct examination, I have found that asking open-ended questions is more effective as it allows the experienced expert to be the center of the jurors’ attention; allows the expert to fully describe his/her qualifications, opinions and conclusions; and therefore inoculates the expert from any claim by the opposition that his/her qualifications or methodology are lacking.

To recap, in order to qualify as an expert, you must demonstrate on direct examination that the witness has special knowledge, skill, experience, training or education on the subject matter at issue. I offer the following checklist of subject areas (consistent with Evid. Code § 720) to help you organize your direct examination:

  • Education (Degrees, Certifications)
  • Professional Experience
  • Specialized Training
  • Teaching Experience
  • Research Experience
  • Publications
  • Special Recognition in Their Field (Honors, Awards)
  • Membership in Professional Organizations (including any leadership positions)
  • Prior Expert Witness Qualifications

You should compare this checklist with the expert’s CV (curriculum vitae or résumé) to make sure that you wring out all the expert’s background relevant to the subject matter of his or her testimony. Also, if the expert has any publications listed on his/her CV that are relevant to the proceedings, make sure you read the articles and discuss them with the expert to see if there is any additional useful information that can be discussed on direct, or any harmful information that can be used by your opponent on cross-examination.

III. Expert Testimony Must Relate to a Subject “Beyond the Common Experience” of the Trier of Fact:

Even if qualified, the expert’s opinions are limited to those, “Related to a subject that is sufficiently beyond common experience that the opinion of an expert would assist the trier of fact…” Evid. Code § 801(a) (emphasis added). This is sometimes referred to as “beyond the ken of a layperson.” For example, doing a simple mathematical equation of adding numbers would not need expert testimony, but a forensic examination of a company’s books may require it.

Typically, the standard of care of a professional will require expert testimony, Avivi v. Centro Medico Urgente Medical Center, 159 Cal. App. 4th 463, 467 (2008); Webster v. Claremont Yoga, 26 Cal. App. 5th 284, 288 (2018), except where the circumstances fall under the “realm of common knowledge,” i.e., a situation that would be so familiar to a lay person that expert testimony is not necessary for the jurors’ understanding.  Sanchez v. Brooke, 204 Cal. App. 4th 126, 138 (2012).

IV. Does the Opposing Expert Pass the Sargon Test?

The trial court serves as a “gatekeeper” to exclude expert testimony that is (1) based on matter of a type on which an expert may not reasonably rely; or (2) based on reasons unsupported by the material on which the expert relies; or (3) is speculative. Sargon Enterprises, Inc. v. University of Southern California, 55 Cal. 4th 747, 771-772 (2012).

In summary, an expert may not base his/her opinions on facts that are not supported by the evidence or are so speculative that they have no evidentiary value. As you prepare for cross-examination, make sure you consider the holding in Sargon to exclude or strike the opposing expert’s testimony.

If you doubt the opposing expert has sufficient expertise to testify as an expert on a specific subject, you may ask the court to conduct a hearing, outside the presence of the jury, to challenge the expert’s qualifications. Evid. Code § 402. If you are successful in demonstrating the lack of expertise, the court will not allow the witness to testify as an expert pursuant to Evidence Code § 720(a) which provides: Against the objection of a party, such special knowledge, skill, experience, training, or education must be shown before the witness may testify as an expert.” (emphasis added). After an Evidence Code § 402 hearing, the court may not strike the expert’s testimony, but may limit the expert’s opinions to those areas where the expert may have the requisite qualifications.

If you have doubts about the quality of the opposing expert, you will need to make a strategic decision as to how and whether you will challenge the expert before trial. If you have a realistic chance at keeping the expert off the stand, an Evidence Code § 402 hearing would be appropriate. If it is doubtful you can preclude his/her testimony, you will be better off not previewing your cross-examination in a Evidence Code § 402 hearing. This strategy will deprive the opposing expert of the benefit of knowing how you will confront them.

V. Invest in Planning Your Cross-Examination:

In planning your cross-examination, there are typically four areas to explore:

1.Qualifications: Is the opposing expert qualified to testify as to the subject matter at issue in the litigation? For example, a traffic collision reconstructionist is not a traffic engineer and may lack the qualifications to render opinions on the engineering aspects of a roadway.

You may wish to compare and contrast the qualifications of your expert with those of the opposing expert. For example, the opposing expert may have experience in research projects involving patients with a certain disease. However, while your doctor does not have that research experience, he/she may have significant experience actually treating patients with the disease. So, for example, cross-examination of the opposing expert will reveal that it has been a decade since they actually treated any patient with the disease at issue, thus, undermining his credibility.

2. Work Product: Did the opposing expert perform the work necessary to render credible opinions? For example, has the expert reviewed the necessary documents in the file, examined the physical evidence, conducted a sight inspection, photographed and measured the objects or scene, conducted tests, documented results, made proper calculations, consulted treatises and textbooks, and considered all evidence necessary to form credible opinions? If not, the expert is subject to doubt.

 Listen carefully when opposing counsel elicits on direct examination the documents his/her expert reviewed. If opposing counsel withheld documents from the expert, the expert’s opinions may be fatally flawed and opposing counsel’s credibility will be seriously damaged.

 3. Logical Opinions: Do the opinions pass the Sargon, supra, test? Do the opinions logically follow from the evidence reviewed? Pointing out failures in logic can make for a devastating cross-examination.

 4. Bias: Does the expert have any bias that would taint the opinions offered? For example, if the expert has a financial bias from regularly testifying for a certain company, firm or side, jurors may conclude that the expert is a hired gun and not worthy of belief.

During cross-examination of the opposing expert you will use leading questions to control the witness. If the witness is not responsive to your question or seeks to add an explanation not called for by the question, object as being “non-responsive.” If the judge sustains your objection, ask that the non-responsive portion be stricken. Evid. Code § 766.

Before you attempt to cross-examine the opposing expert, do all you can to educate yourself in the subject matter of the expert’s testimony. Make sure you understand and can properly pronounce key terms and phrases. Meet with your own expert and have them school you in the subject matter. If they have suggested reading, invest the time to learn. Review the opposing expert’s report with your witness and seek their help as to any holes in the opposing expert’s work product, methodology, or any leaps in logic.

While a witness must always be responsive to the question asked, experienced witnesses know how to counterpunch. I have seen experts frustrated with answering “yes” or “no” to a series of questions, lay a trap for counsel by saying, “Well counsel, that is partially true.” Inexperienced counsel will then blurt out, “how is that not fully true?” By asking this open-ended question, the expert is now free to pontificate and explain.

Evidence Code § 721(a) governs the cross-examination of expert witnesses. It permits cross-examination of the expert’s qualifications, the subject matter of his/her opinions, and the basis for each opinion.

Evidence Code § 721(b) does not permit cross-examination of an expert as to “the content or tenor of any scientific, technical, or professional text, treatise, journal, or similar publication…” except in specified circumstances. But here is an opportunity that is too often overlooked. The contents or tenor of a text, treatise, journal or other similar publication may be used in cross examination in three circumstances:

Evidence Code § 721(b)(1) if the expert relied upon, referred to, or considered the publication in forming his/her opinion.

Evidence Code § 721(b)(2) if the publication “has been admitted into evidence.”

Evidence Code § 721(b)(3) if “the publication has been established as a reliable authority by the testimony or admission of the witness or by other expert testimony or by judicial notice.”

Evidence Code § 721(b) can be a powerful weapon on cross-examination if used correctly. For example, if you find a text, treatise or journal that contains information contradicting the opposing expert’s opinion, have the expert admit in deposition that the text is an authoritative source. See, Evid. Code § 721(b)(3). During cross-examination at trial, get the expert to make the same admission. A sample cross-examination in a medical malpractice wrongful death case alleging a failure to diagnose septicemia (blood poisoning) might proceed as follows:

Counsel:     Doctor, you agree that Mandell’s textbook Principles and Practice of Infectious Diseases is an authoritative textbook on infectious diseases?

Doctor:       Yes.

Counsel:     In fact, Mandell’s book was the textbook you used in medical school while learning about infectious diseases.

Doctor:       Yes.

Counsel:     You agree with me that Mandell’s textbook is commonly relied upon by doctors in formulating their diagnoses of infectious diseases.

Doctor:       Yes.

Counsel:     In fact, you routinely consult Mandell’s textbook in making your own diagnoses.

Doctor:       Yes.

Counsel:     You did not see fit to consult with Mandell’s text in formulating your opinions in this case however.

Doctor:       True.

Counsel:     You are aware that Mandell’s text lists the signs and symptoms of common infectious diseases?

Doctor:       Yes.

Counsel:     You are aware that Mandell’s text lists the signs and symptoms of septicemia?

Doctor:       Yes.

Counsel:     And those signs and symptoms include (counsel lists the signs and symptoms of septicemia)?

Doctor:       Yes

Counsel:     And the decedent exhibited the following signs and symptoms when you examined him at the Emergency Department on July 5th of last year: (Counsel lists the signs and symptoms the decedent exhibited at the ED).

Doctor:       Yes.

Counsel:     All of those signs and symptoms are consistent with a diagnosis of septicemia as instructed by Mandell’s textbook on Infectious Diseases?

Doctor:       Yes.


A skilled expert witness will lend credibility to your arguments and bolster your case by advocating positions consistent with your case theme and theory. The best experts are good teachers who can simplify and explain the evidence to jurors in an understandable way. Make sure you showcase your expert’s qualifications on direct examination and limit his/her testimony to the subject matter of their expertise.

If you are opposing the proffered expert, invest time in preparing for cross-examination. If the opposing expert is not qualified in a particular subject matter or bases his/her opinions on improper or speculative matters, seek to disqualify the expert pursuant to Sargon, supra, using an Evidence Code § 402 hearing. Or consider Evidence Code § 721(b) to undermine an opposing expert by showing his/her opinion is contradicted by reliable authorities.

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