In the context of a civil or criminal court case, a witness is a person who answers questions under oath during a deposition or a trial.  The statements of each witness are his testimony, which is one type of evidence that the trier of fact (judge or jury) may consider in reaching its ultimate decision about what happened.  Significantly, there are several types of witnesses—including expert witnesses—with the rules regarding testimony depending on the type.  In particular, this article explains why opinions are allowed to be included in expert witness testimony.

Eyewitness Testimony

If you are called to serve as a juror, the first type of witness you may encounter is a fact witness.  A fact witness is a person who testifies about what happened from their point of view.  As a general rule, fact witnesses should not offer any opinions.  Instead they should limit their testimony to their first-hand knowledge.  This usually means what they observed or witnessed through one or more of their five senses plus their recollections of their own actions.  For this reason we may refer to such a person also as an eyewitness.

For example, one fact witness in a murder trial may testify that on the night in question she saw the accused person enter the building at about 9:00pm, that a short time later she heard a fight and then a scream, and that finally she saw the accused run from the building and hurry off down the street about 15 minutes after he had arrived.  Another neighbor might testify as a fact witness that he heard the fight and the scream and called 911 at about 9:15pm. 

A police officer might also be a fact witness, offering testimony about what he saw and smelled when he entered the apartment and found the dead body.  The police officer's testimony might also include that he personally took the photographs at the same address that evening and wrote the report that was officially filed by the police.

Sometimes fact witnesses are called simply for the purpose of authenticating one or more documents so that they can be entered as evidence.  The police officer does this by speaking of the history of the photographs and police report, as one example. 

Opinions and Expert Testimony

In United States District Courts the inclusion of opinions in testimony is regulated by Article VII of the Federal Rules of Evidence, which standardize the admissibility of evidence in federal civil and criminal trials.  Rule 701 regulates the opinion testimony of so-called lay witnesses while Rules 702-706 regulate expert witness testimony.

A lay witness is a non-expert who is providing an opinion.  Non-experts may only offer the court an opinion that is:

  • "rationally based on the witness's perception;
  • helpful to clearly understanding the witness's testimony or to determining a fact in issue; and
  • not based on scientific, technical, or other specialized knowledge"

For example, a non-expert might be permitted to opine that when he saw the accused enter the apartment he seemed angry.  This is an opinion that any reasonable person might have (i.e., one need not be a scientist to see anger in body language) that is based on their perception.  That said, depending on various factors, a judge may or may not allow an opinion of this sort to be heard by the jury in a particular case.

An expert witness is a special type of person that is called to provide expert witness testimony.  For example, a DNA expert might be called to testify about skin cells found under the victim's fingernails that link the accused to the crime with a very high probability.  Most of the time an expert witness will be an independent professional who has no prior connection to either of the parties or the facts of the specific case.  Unlike the fact witnesses, an expert witness is generally paid for his work including for time spent at the trial to give his testimony.

Before a witness can provide any expert testimony, he must first be accepted by the judge as qualified by way of his "knowledge, skill, experience, training, or education" in a particular "scientific, technical, or other specialized" area of expertise.  Barr Group co-founder Michael Barr, for one example, has been qualified by various judges as an expert in specialized fields of knowledge ranging from computer security to electrical engineering to satellite TV piracy

Under Rule 702, opinion testimony is permissible from a qualified expert when:

  • "the expert's scientific, technical, or other specialized knowledge will help the trier of fact to understand the evidence or to determine a fact in issue;
  • the testimony is based on sufficient facts or data;
  • the testimony is the product of reliable principles and methods; and
  • the expert has reliably applied the principles and methods to the facts of the case."

Often, at least in the adversarial American legal system, both sides will present their own experts who have reached somewhat or totally different opinions based on the same underlying facts and evidence.  In such a case it falls to the trier of fact to weigh the opinions of the competing experts.

Fact Witnesses vs Expert Witnesses

Generally, then, we can summarize that there are just two types of witnesses:

  • Fact witnesses describe what they observed or did along with either no opinions or very limited "lay opinions", which are facts for the judge or jury to consider.
  • Expert witnesses provide "expert witness testimony," sometimes containing extensive opinion to help the judge or jury understand and make sense of the other evidence in the case including even the testimony of fact witnesses.

Though an expert witness might also act as a fact witness in a portion of his role in a particular trial (describing his own observations and actions at the scene of a crime, e.g.), a fact witness is not someone who would also be presented to the court as an expert.

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