Witnesses come in two varieties: Lay witnesses and expert witnesses. Lay witnesses testify only when they have personal knowledge about the matter about which they have been offered as a witness. And, generally, they testify about facts, i.e., what they have seen, heard, or read. Lay witnesses can only offer opinions only when they are “rationally based on their perceptions” or “helpful to clearly understand the witness’s testimony or determine a fact in issue.” Lay witness testimony cannot rely on scientific, technical, or other specialized knowledge.

Expert witnesses, on the other hand, rarely have first-hand knowledge about the facts associated with a case. Instead, they rely on what they are told, and on what they read. They use their “knowledge, skill, experience, training, or education” to offer opinions based on the facts as they have been shared with them.

Expert witnesses come in many varieties. There are people who look at the remains of an automobile collision and testify about how fast people were driving. Medical professionals talk about whether a doctor or nurse met the standard of care in providing care, and can also provide information about whether what the doctor or nurse did affected the outcome. Many years ago I represented a man with a marketing/economics background who provided expert testimony for auto dealers in franchise disputes with manufacturers.

From time to time I provide expert testimony on legal matters. My testimony usually involves the reasonableness of fees charged by attorneys, or whether an attorney has met the standard of care when he or she provided legal services. I testify for one side or the other in any given case, but I have testified for both attorneys and clients. Keeping some balance between providing testimony for attorneys or clients helps with credibility issues, as the expert who only testifies for one side, after a while, becomes less believable, even if he or she may be right.

One big difference between lay and expert witnesses relates to payments. Lay witnesses receive a nominal fee for their time, while expert witnesses get paid at professional hourly rates. (Those fees can be very high.) Professionals, providing fact testimony, only get the statutory witness fee, as one attorney I knew learned many years ago. The attorney had worked for my client, and was let go. He had to provide testimony about a case he had worked on, and wanted to be paid at his hourly rate. “No dice,” said the judge, as he was providing facts and nothing more.

Expert testimony is controversial. Many—in particular, corporate defendants—believe courts allow expert witnesses to offer opinions based on “junk science” in support of personal injury claims. (Many attorneys believe junk science is whatever the other side’s expert offers as a theory.) In Daubert v. Merrell Dow Pharmaceuticals, Inc., No. 92-102, the U.S. Supreme Court established standards to guide trial judges in determining whether a jury hears expert testimony. Subsequent cases have refined and clarified the rules for trial judges, but in any given case disputes still arise about whether and to what extent and expert witness can offer opinions.

Cases depend on facts, and we get facts from documents and people, generally. Expert testimony is part of the process because, despite the flaws, the system knows jurors do not always have an adequate knowledge base to process facts. The system assumes, and it’s generally a valid assumption, that jurors will hear from competing experts and use what they hear to give the facts appropriate context.