What is an Expert witness?

While a fact witness is actor in a case and provides information about the facts he is aware of, an expert witness is a third party who expresses his independent opinion based on the information that he was provided with.

In the United States, an expert is hired by either party to a litigation (or sometimes the court) to help understand the case. He must be "qualified by knowledge, skill, experience or education" on the topic of testimony.

An expert's testimony must be "based on sufficient facts, data or products", and must be "based by verifiable methods and principles". Experts must produce a written report, and may be compelled to deposition.

Why hire an expert witness?

In the US, an expert witness is called by either party to "help the trier of facts to understand the evidence or to determine a fact in issue”. We can complement this rule by explaining why an expert witness contributes to a case.

  • Nowadays more than ever, the testimony of an expert witness will often “make or break” a complicated case, where technical knowledge is critical. Litigation attorneys therefore try to find an expert witness who understands the complex issues involved and who, most importantly, can explain these issues in simple terms to a jury or panel.

  • The expert will also guide an attorney in the critical aspects of the case, will spot where the standards and practices have been violated, or will point the attention to the weakness of an argumentation. You surely do not want to discover late in the process (or in court) that a critical issue has not been investigated and argumented.

It is also recommended to take your exper to depsoitions and trial, as he will pick up on professional jargon and expose limited explanations. He will dissect opposing testimony and suggest questions. An expert will change the impression left by the opposing side.

When hire an expert witness?

Many cases require the help of an expert witness months prior to their testimony. Moreover, finding the right expert takes time. So it is prudent to start the selection process early on. Actually, multiple bar associations and professional institutes recommend the hiring of an expert as early as possible - even before litigation has started.

An expert can guide you in the early stage of your case. Even before claims have been filed or demand letter exchanged, he can guide you in your dialogue with the opposite party to extract as much information as possible. He will then help writing the initial claim or answer one. He will guide in the discovery process.


An expert witness needs time to prepare for your case. He'll need to review all pertinent documentation, work with the client and the attorney to ensure that they understand all aspects of the case. He may need to make research on his own. He needs to write a report, the basis of his testimony. If a deposition is required, additional preparations and time allotments also are required.

Last but not least, because the number of capable experts are limited in highly specialized areas of testimony, you may want to attract and bind the best expert early on in your process.

How to select an expert witness - the "C's"

The main points to consider when hiring an expert are

  • Conflict: make sure your expert has no conflict of interest with either interested parties and that he is available for your case before introducing any detail.

  • Competence: is the expert really qualified in the area? Like all consultants, some less reputable experts may be tempted to extend the limit of their knowledge.

  • Credibility: is the expert credible, by his background or his knowledge of the facts? Qualifications do matter, in court probably more than anywhere else. While the experience an expert brings is valuable to understand the case, his professional pedgree and prestige will also influence the psyche of the jury/panel. This being said, any expert with a high opinion of himself may be irritating to his listeners.

  • Communication: you want your expert to have gravitas in court, but intelligent people are sometimes not that great at communicating thoughts to a lay person. An outstanding resume is not nearly enough. Often, the deciding factor in selecting an expert is how he will present to a finder of fact. Personality is important, including the perception of impartiality and objectivity.

  • Court experience: a first-timer may be a risk, especially on the stress of a cross-examination, but a polished expert has its drawbacks too. Juries may not like somebody who is a recurrent "hired gun". Worse, because of multiple previous testimonies, your expert may be tarnhished by an opposing case. There is a sweet spot to find. Some expert with little testimony experience are naturally great at explaining or convincing a jury and do very well on the stand. 

  • Continuity: make sure that your expert is willing to see the case through the end, or that he is affilated with groups who will take the case over if needed. Does the expert have a team to support him if the workload increases?

Now, like for all hire or contractual agreement, do your homework.

  • Investigate your expert, starting by requesting reports and publications: your opposite party will surely do it, so don't forget to do it first. Ask for references and call them. The last thing you want is discover in court that your expert has some 'baggage'.

  • Meet or interview the expert in person: His demeanor and appearance are extremely important. Is he shy, nervous, arrogant? Confidence and professionalism are required in court, so does he show it in your conversation? How prepared is he for the meeting? Is he energetic and enthusiastic? How well does he answer tough questions? 

Further references

Here are a list of references or further analysis.