An "opinion" is "an inference or conclusion drawn ...from facts some of which are known to him and others assumed, or drawn from facts which, though lending probability to the inference, do not evolve it by a process of absolutely necessary reasoning"
When a witness is put on the stand their primary role is to recall their knowledge of some past event at issue in the case. As a general rule no witnesses are allowed to give any opinion. However, in practice, the line between fact and opinion can never be clearly drawn, so often exceptions will be made. Opinion, in certain cases, can be useful to the trier of fact in coming to their conclusions. Experts are generally allowed to give their opinion in matters within their expert knowledge that the trier cannot easily deduce themselves.
To address these situations the law of opinion is divided into three categories of opinion evidence that is admissible:
- lay opinion,
- expert opinion, and
- statutory or regulatory certified experts.