September 18, 2014
There are two types of witnesses - - fact witnesses (sometimes called percipient witnesses or eyewitnesses) and expert witnesses.
Most witnesses are so-called fact witnesses and can testify as to what they personally saw, heard, or experienced. For example, a witness to a car accident can testify about what he personally saw. But could he testify as to whether the car that was involved in the accident was properly designed? Probably not. Even if he has an opinion, a judge won't allow him to testify because his lay opinion will not help the jury.
To testify about car design and many other topics that require specialized knowledge, one must be an expert. Experts can provide opinion testimony about matters that ordinary people cannot. Think about television crime dramas where medical professionals testify about the time and cause of death even though they did not see the victim die.
Before a witness can testify as an expert he has to satisfy several criteria. If you look at Federal Rule of Evidence 702 you can see the standards in the federal courts for a witness to testify as an expert. First, the judge will have to be satisfied that the witness is actually an expert based upon education, training, knowledge, or skill. For example, a doctor or a medical researcher might have the appropriate education and training to testify about a new drug.
Second, the judge must be satisfied that the expert's testimony is based upon sufficient facts or data and that the expert applied the proper principles to the facts or data to reach his conclusion. It's not enough for a doctor to say that he believes a certain type of medicine was inappropriate to treat a patient. The doctor must satisfy the judge that his analysis was sufficiently rigorous.
Finally, if the judge agrees that the witness is an expert, and that the expert applied the proper principles and methods to the facts and data to reach his conclusion, the witness will be allowed to testify if the judge believes that the expert's testimony will be helpful to the jury.
I uploaded a video to my Youtube page on expert witnesses: