OPINION EVIDENCE

Generally only evidence of fact is admissible and opinion is not. Opinion evidence may be given in certain circumstances by lay witnesses and in others by expert witnesses.

Lay witnesses
Lay witnesses can give opinion evidence if one of the disputed facts is the opinion itself. Alternatively it will be admitted if it would result in significant impracticality if the court did not allow it.

There are a few things which are acceptable for lay witnesses to give evidence about.
 * Identity of people, handwriting and other things (See IDENTIFICATION EVIDENCE )
 * Apparent age
 * Body build and appearance
 * Another’s emotional state
 * An item’s condition
 * Speed and distance estimates

Where the witness is of a considerable age and experience they can give opinions as to whether someone was drunk.

Expert Witnesses
Whether an expert can give evidence on a particular matter depends on two rules.
 * 1) Whether the evidence is common knowledge and thus the jury does not need further information to be able to decide on the matter.
 * 2) Whether there is an accepted area of knowledge from which the evidence comes.
 * Such a body of knowledge must be sufficiently accepted by those studying the subject.
 * The expert must be qualified in the particular area. This may be through formal qualifications and/or sufficient experience.

The opinion must be shown to be based on fact.Experts can speak about probability but not in strict mathematical terms unless they are a statistician.

The hearsay rule
The hearsay rule (see HEARSAY AND ITS EXCEPTIONS) is relevant to expert opinion evidence
 * Primary facts given by the expert are subject to the hearsay rule (Example of a primary fact: The car that was damaged in this scenario was blue).
 * However the hearsay rule does not apply to facts of general application. The expert must tell the court that the source of that information is another. (Example of fact of general application: In Western Australia 45% of blue cars are owned by women).


 * Experts can comment on what they believe the ultimate factual conclusion to be but cannot do the same for the ultimate legal outcome (e.g. An expert can state (if they have the relevant expertise) that the glass of the car windscreen was shattered due to a being struck with force by an implement. They cannot however state that in their opinion the accused is guilty of the offence of criminal damage).