Limits of Expert Testimony: Staying Within Scope

Expert witnesses play a unique role in court cases.

Edward Maggio, J.D., M.S.

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— Updated on June 29, 2022

Limits of Expert Testimony: Staying Within Scope

Based on their credentials, professional experience, and review of evidence presented at trial, experts can offer testimony regarding events they did not witness firsthand. This is generally acceptable in personal injury cases, mass tort matters, or in trials involving accident reconstruction. However, expert witness testimony does have limitations. For trial attorneys, a lack of awareness of these limitations can be problematic.

The case of Roy v. Dackman illustrates the importance of identifying any potential limitations when it comes to an expert’s credentials or the evidence relied upon by the expert for their testimony. Ultimately, credentials and evidence form a tight-knit benchmark for ensuring an expert witness’s testimony carries with it general acceptance and persuasiveness.

Case Study: Roy v. Dackman

Roy v. Dackman illustrates how there can be a disconnect between an expert’s credentials and the evidence. In this matter, the Court of Appeals of Maryland reviewed the involvement of a pediatrician testifying as an expert in a lead poisoning case. At trial, the pediatrician was found unqualified to testify. On review, the Court of Appeals stated that the issue was whether a pediatrician, who had never treated a victim of lead paint poisoning but claimed to have read the relevant medical literature, should be permitted to offer an expert opinion as to medical causation.

The Court held that based on their credentials, the pediatrician was competent to testify as an expert as to medical issues. The court noted:

“A witness may qualify [as an expert] if he possesses special and sufficient knowledge regardless of whether such knowledge was obtained from study, observation or experience…There are many expert astronauts who have yet to make a space flight.”

However, the Court also held that with no personal knowledge of lead poisoning combined with only relying on circumstantial evidence as to how the plaintiff was exposed to lead paint, the plaintiff’s expert was not competent to testify as to the source of the lead. Put another way, as a pediatrician, the expert could testify regarding how lead poisoning medically affects children. However, the expert witness could not testify that lead was the specific cause of the plaintiff’s injuries. The evidence relied upon by the expert led to them being out of scope with their testimony.

How an Expert’s Credentials Relate to the Evidence

The credentials of an expert witness must be understood in order to ensure they stay within accepted limits. Prior to ever stepping into a courtroom, an expert’s professional experience, education, and career background lay the foundation for acceptance of their testimony. Many jurisdictions will require an expert witness to provide such information in advance. This includes:


An expert’s professional education is a critical part of their credibility. This also includes any professional training they have received within their field or profession that is relevant to their upcoming testimony. As shown in Roy v. Dackman, a medical doctor can certainly testify about general medical matters. This sets a standard that an expert with advanced degrees in their field is not likely to face challenges when giving testimony of a general nature.

Career Advancement

Generally from a resume or interview with the expert witness, an attorney can learn key information. Attorneys can learn how the expert’s career progressed over a period of many years. This includes areas of research and publications which support future testimony by the expert witness. This allows an expert to develop unique personal knowledge about a subset of their field. Although the expert in Roy v. Dackman had plenty of career experience in pediatrics, they were shown to have no personal knowledge related to lead poisoning. This was a key point in blocking the expert from providing any testimony as to that specific subset of medical information.

Continued Study and Peer Involvement

Expert witnesses who stay current with developments in their field continually participate in industry events with their peers. Every field has new developments over time and new views on major key subjects. Conferences or peer-related functions are the perfect way for an expert to keep up to date. In addition, conferences allow experts to stay at the forefront of their field of expertise on a topic.


In preparation for trial, and during actual testimony, the expert witness needs to be able to demonstrate their reliance on the most appropriate evidence. In most cases, an expert witness can testify to:

  • The data, information, records, or case studies reviewed prior to their testimony concerning the case. This includes any questions asked by counsel during their review of the evidence.
  • The specific methods used to analyze information and how they arrived at their expert opinion. This can include any research reviews, laboratory testing, or a medical examination.
  • Whether the methods in use by the expert are common in their chosen profession and for their field.
  • The final opinion or conclusion that the expert witness researched as a result of their efforts.

The reliance by an expert on circumstantial evidence or a complete lack of review of any evidence is concerning. Such evidence is ripe for a challenge by the opposition. For the expert in Roy v. Dackman, this proved to be the final nail in the coffin as to the admissibility of a key part of their testimony. When it comes to complete admissibility of expert testimony, the evidence must be substantial. The expert must conduct first-hand reviews of any evidence directly prior to any testimony.

In terms of limits, experts may give their opinions or inferences that address an issue in a case. For example, an expert may testify that it is their opinion that exposure to a specific chemical was a possible cause of the plaintiff’s illness. However, this requires support by a foundation of substantial evidence.

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