Opinion Evidence
One of the difficulties about trying to explain aspects of the law of evidence for the layman is the inherent complexity of the subject and the difficulty of making generalisations. There is no point saying something in such a way that one has to be an expert to follow it. One has to guard however against the problem that attempts to simplify can end up being to a greater or lesser degree inaccurate.
I have decided to risk the dangers of a degree of inaccuracy in the interests of clarity and simplicity. Making something too complicated will be of no use as an introduction to the subject for the layman interested in the law. However in the law of evidence even more than in various other branches of the law, a general proposition may be subject to a series of exceptions. If you ever decide to try and explore the subject in more depth, please bear this in mind. Do not assume there are no exceptions to the general rules that I set out.
Generally witnesses are not allowed to give opinion evidence. The main exception to this is expert evidence. The expert is qualified in the particular sphere, it is an area relevant to issues in the case, and the court considers that the evidence will potentially be of assistance. I shall deal next week with the topic of expert evidence. However in general unless you are an expert, called as an expert, you cannot give opinion evidence.
Witnesses are supposed to give evidence as to facts. The principle is supposed to be that facts are susceptible of proof, of consideration, of contradiction. Opinion could be based on more elusive matters, and ultimately involves decisions about facts which are really for either the Judge or the Jury not the witness.
Thus far the principle is easy to state. However often there is no fine line between fact and opinion. In practice opinion may have to be admitted if it simply cannot be easily separated out from comment as to fact. This can arise in a number of different contexts. For example in a driving case the witness says that a car was on the wrong side of the road and went through a red light. Although "wrong" might sound like opinion, insofar as it is shorthand for the car was driving on the right hand side of the road it is a statement of fact. However if he says in his opinion it was really dangerous that is opinion. However often it is not so clear cut. If he adds that the car was going "really fast", (permissible), he is stating that as a fact but it involves an opinion (unless he is sitting in the car staring at the speedometer). It may also be impossible to specify matters of fact without condescending to opinion, or they may be inextricably linked with matters of fact. (" as he came towards the table, he looked aggressive and I thought he was about to attack me, so I shouted for help").
Therefore in general the witness cannot state his opinion, but if it is necessarily and inextricably linked up with the questions of fact then that is likely to be permissible. As we shall see in many areas of evidence, it is often difficult to know exactly where to draw the line.