The Federal Courts rely on a single set of rules, and associated court interpretations and rulings, known as the Federal Rules of Evidence, to determine the admissibility or inadmissibility of evidence. The Federal Rules of Evidence are based in large measure on traditional English Common Law, with some additions that have been made over the years. In the spirit of common law, any court can reinterpret the rules for a particular situation (assuming a higher court has not implicitly or explicitly offered a contradictory interpretation under similar circumstances). Thus, a complete set of Federal Rules of Evidence is often brief, as it consists only of the United States Code that pertains to the Federal Rules of Evidence. By contrast, the annotated edition – complete with detailed explanations of what each rule means under a host of different circumstances – can fill an entire book.
The rules governing the testimony of experts are collectively referred to as the 700 rules, or the expert testimony rules. Because every rule is numbered, and rules on a topic are often given the same first number (in this case, 7), the expert rules are all designated by three digit number starting with 7. The most important one is 702, which defines an expert as any witness who is qualified by experience, skill, training, or education to testify on topics that a layman cannot. Generally, the standard is that the expert must have experience, skill, training or education that provides him with a better understanding of the topic than would be available to the average person.
Rule 702 ties in significantly with 701. 701 bars lay witness from offering testimony beyond that which their rational perception provides them with. Thus, under Rule 702, a type of witness is provide for who can provide testimony beyond that which his rational perception provides him with. Of course, the expert must disclose the basis of his expert opinions, if the court requires them. Generally, the requirement is that the basis of his opinion be facts, data, or methods that are commonly used or commonly available in his field.
Rule 704 forbids an expert in a criminal case from testifying to “the ultimate issue.” This means that the expert may testify that “John pulled the trigger and that led to the death of Jane” but not that “John murdered Jane.” The former is a statement of fact – a sequence of actions. The latter is a legal conclusion, and the expert is not allowed to testify to this, as it is the jury’s prerogative.