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The Dangers of Expert Witnesses Offering Net Opinions

A net opinion summarizes an expert’s opinions and conclusions without explaining the facts or reasoning the expert used to reach those ends.

Dani Alexis Ryskamp, J.D.

Written by
— Updated on October 19, 2022

The Dangers of Expert Witnesses Offering Net Opinions

Net opinions can be a useful shorthand for attorneys as they work through potential arguments. However, when an expert’s conclusion is favorable to one’s client, it can also be tempting to submit the net opinion on its own. Expert witness net opinions can be dangerous when offered to a court in place of an organized expert witness report.

Understanding the Admissibility of Expert Witness Net Opinions

The Federal Rules of Evidence, and most state rules of evidence, do not explicitly bar the use of net opinions in court. The expert witness testimony standards in Daubert do not explicitly bar the use of net opinions either. Why, then, do courts frown on them?

Typically, courts that bar net opinions derive their rule from Federal Rule of Evidence 702. This rule requires that expert testimony be “based on sufficient facts or data” and “the product of reliable principles and methods.” Courts tend to reason that if Rule 702 requires experts’ opinions and conclusions to be based on facts, data, principles, and methods, then an expression of these factors is required—particularly as Rule 702 requires an evaluation that facts and data are “sufficient” and principles and methods are “reliable.”

The Net Opinion in (In)Action

While unpublished, the opinion in Philadelphia Contributionship Ins. Co. v. Ryan, Inc., a New Jersey case, can shed light on the problems posed by net opinions. In Ryan, a fire occurred in a fuel oil furnace shortly after it was serviced by one of Ryan’s technicians. Philadelphia Contributionship Insurance Company paid the homeowner’s claim then sought to recover that sum from Ryan, alleging that Ryan’s technician was negligent in his repair of the furnace.

During discovery, the plaintiff amended their answers to interrogatories to include summaries of two oral reports from expert witnesses. The trial court ruled that both summaries were inadmissible net opinions. The summaries included each expert’s conclusions, but they failed to explain the “whys and wherefores” that led to those conclusions, according to the trial court.

The appellate court affirmed, stating that the experts’ opinions were impermissible net opinions because they did not explain the methodology by which the experts’ conclusions were reached. Because the summaries lacked this information, the court said, “…it cannot be said that anything in [the expert’s] report constituted ‘specialized knowledge [that] will assist the trier of fact.’”

Expert Responsibilities: Show Your Work

One way to explain the net opinion problem is to liken it to a math problem. Just as teachers often expect students to “show their work” when solving an equation, courts expect experts to also “show their work” by laying out the evidence and methods that informed their conclusions.

An expert’s opinion cannot be based on no stated facts, data, principles or methods, just as the “show your work” requirement in math class can’t be met by simply writing down the answer. Experts do have, however, some leeway. They are allowed to weigh types of evidence and choose reasoning methods according to the demands of their area of knowledge.

For example, in Interlante v. Target Corp., the U.S. District Court for the District of New Jersey stated that an expert’s opinion may be based on circumstantial evidence, which the court defined as “a preponderance of probabilities, according to the common experience of mankind.”

According to this decision, experts may make inferences in their opinions and conclusions, so long as the premises of those inferences are explained. For instance, in Interlante, the court found that the expert witness’s opinion was admissible because it was based on information in an incident report, an inspection of the premises, and an examination of what the plaintiff was wearing at the time of the fall that caused their injuries.

Although the expert in Interlante made certain inferences from the report and his own observations, the opinion did not cross the line because the inferences were explained in a way that allowed the court to understand the reasoning. The court found that the expert’s opinion was not, therefore, a “net opinion.”

As long as an expert lays out the facts and reasoning they used to reach their conclusions, the net opinion rule will generally not apply. Attorneys can protect the admissibility of an expert’s opinion by encouraging experts to “show their work” in a clear, organized fashion aiding the trier of fact.

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