Understanding the Scope and Limitations of Expert Opinions

Here, we explore the scope and limitations of expert opinions, including what experts may testify to, on what information they can base their testimony, how confident experts need to be in the “correctness” of their opinions, and the types of cases in which experts most commonly appear.

What is an Expert Opinion?

An expert opinion is a specialized conclusion or inference drawn by a qualified individual in their field of expertise. Unlike lay opinions, expert opinions are based on specialized knowledge, training, or experience and are used to help the fact-finder understand complex issues in a case. These opinions must be grounded in reliable methods and data commonly relied upon by professionals in the same field.

What Can Expert Opinions Contain?

Some limitations exist, however. For example, an expert is generally expected to base an opinion or inference on facts or data that are reasonably relied upon by similar experts in the subject area. When an expert presents certain facts, it is important to establish how those facts support the opinion the expert expresses.

Experts can receive information about the case on which to base their opinion. In fact, this information is often essential: without it, the expert may be able to form no opinion at all. The resulting opinion must typically be based on available facts. Like lay witnesses, experts are not generally allowed to speculate.

While many jurisdictions do not require an expert to list all the facts or data he or she received before forming an opinion, experts will typically be expected to list these if asked to do so on cross-examination.

How Right Must an Expert Be?

Expert opinions are subject to various “correctness” standards in various jurisdictions. For instance, Florida state courts require that the opinion must be stated “within a reasonable degree of probability,” or with at least 51 percent likelihood that the opinion is correct. In practice, ensuring that an expert is as confident as possible in the correctness of his or her opinion can both bolster the value of the opinion itself and the reputation of the expert in the mind of the fact-finder.

What Can An Expert Testify To?

In most cases, an expert witness will be allowed to testify to:

• Their own background and qualifications
• The information or data they received about the case, including any hypothetical questions they were asked to consider
• The methods they used to analyze this information and arrive at an opinion, such as laboratory testing, medical examination, or research
• Whether and how those methods are commonly used in their field
• What opinion, inference, or conclusion the expert came to as a result of their

Experts may give opinions or inferences that address an issue of fact in a case. For example, an expert asked to testify as to whether a particular surgical error could have caused a plaintiff’s nerve damage can testify that it is (or is not) their opinion the error caused the damage. However, the expert may not, as a rule, go on to tell the finder of fact how to rule as a result.

On cross-examination, common areas of attack include the expert’s education, training, practical experience, review of particular materials, information, time spent on the case, and/or bias – or the lack thereof. Here, research and careful preparation of the expert can prove beneficial.

In What Types of Cases Do Experts Commonly Testify?

Expert testimony is common in a wide range of civil cases and in some criminal cases. Expert witnesses are often called upon to provide their opinions in cases involving:

• Medicine: The complexities of modern medical practice result in attorneys’ use of experts to address medical questions surrounding liability, causation, and damages in several fields, including personal injury, medical malpractice and product liability
• Business: Business experts may include forensic accountants, organizational psychologists, and others who specialize in understanding particular commercial ventures and processes
• Agriculture: Certain commercial litigation, personal injury, and nuisance claims, as well as specific business cases, call for experts in areas of the agricultural sciences
• Valuation: The value of real or personal property or of services rendered is a key issue in many contracts cases, business disputes, and will or estate challenges. Here, appraisers and similar experts can prove invaluable
• Accident Reconstruction: Many personal injury, product liability, and premises liability cases benefit from the attention of an expert witness skilled at reconstructing how an accident took place
• Forensic Scientists: Criminal cases often involve the testimony of experts who focus on ballistics, DNA, fingerprint analysis, and similar subject areas

Generally speaking, if an expert’s perspective would help the fact-finder understand an issue in the case, an expert witness is likely to appear. Since the opportunities and limitations an expert faces on the stand vary by jurisdiction, understanding the applicable standards is essential for attorneys who want their expert to appear in the best possible light.