1. What are expert witnesses and lay witnesses?

1. What are expert witnesses and lay witnesses? 

An expert witness is a person who is suitably qualified by knowledge or experience to give opinion evidence on a relevant issue in the proceedings. Under section 58(1) of the Evidence Ordinance (Cap. 8), a person called as a witness in civil proceedings may give an opinion on a relevant matter if he is qualified to give expert evidence on that matter. 

Expert evidence is permitted because the court may need assistance on matters requiring specialised knowledge. An expert may give opinions and draw inferences based on a recognised body of knowledge, such as science, medicine, engineering, accounting, art, valuation, or a specialised trade. 

For example, an expert may give evidence on medical causation, professional standards, valuation, engineering defects, accounting issues or foreign law. 

An expert may also give an opinion on an “ultimate issue” in the case. This means the expert may express a view on an issue that the court itself must ultimately decide. For example, in a professional negligence case, the ultimate issue is often whether a professional fell below the required standard of skill and care. Expert evidence is frequently used to help the court determine what that standard is and whether the defendant's actions met it. 

However, the court may refuse to admit or rely on such evidence if the matter is one that the judge can decide without technical or specialised assistance. 

By contrast, a lay witness is an ordinary witness who does not need any special qualifications. A lay witness usually gives evidence about facts that he personally saw, heard, or experienced. The evidence of a lay witness should generally be confined to factual matters, and the witness should avoid expressing opinions. 

There is, however, a narrow exception. Under section 58(2) of the Evidence Ordinance, a lay witness may only give an opinion where it is a practical way of describing facts personally perceived by that witness, for example, saying that a person appeared “intoxicated” or that a vehicle was travelling “fast”.