Reputation and Opinion
As a general rule, opinion evidence is inadmissible. Witnesses are supposed to testify about facts, not opinions. The witnesses should present the facts, then the jury (trier of fact) can form opinions on the facts.
Compendious statement of facts is an exception to the opinion evidence rule: lay witnesses are permitted to testify in the form of an opinion if, by doing so, he is more able to accurately express the facts he perceived. There are no degrees of expertness in lay opinion : e.g. no one is an expert witness about whether someone is unable to drive, whether the police or not (but do have this for expert evidence). There is a broad spectrum for allowing testimony of lay people (e.g. handwriting, age of goods, moods). No opinion can be given on legal issue.
A lay person is not qualified to testify on a number of subjects to safeguard against speculation. E.g.: Eyewitness identification and item recognition are opinion matters.
The drawing of interferences are generally prohibited in courts (we do not want police officers coming into court and simply saying "in my opinion, the accused was driving under the influence of alcohol".
Lay opinion evidence is admissible to identify a person or a thing.
There are two exceptions to the rule disallowing opinion evidence: common knowledge and expert opinion.
First Exception to the Rule Disallowing Opinion Evidence: Common Knowledge
Lay opinion (from the accused, experts or outside expertise) can be used if it is part of a shared human experience. Usually a situation where witness cannot explain themselves clearly without using the opinion. It can be argued that opinion evidence can be given provided that it does not usurp the ultimate issue for the trier of fact (i.e.: not conclusive).
Eyewitness identification is the identification of the accused either visually or in any other sensory manner. During the pre-trial phase, it can be completed in various forms: photo line-up or police sketch. However, these can be weakened by "unconscious transfer" where the police or media convey a perception of guilt onto the accused.
Docks identifications are generally weak and the weaknesses can be exploited in cross examination. In the United States and in other jurisdictions, they allow expert evidence on weaknesses of relying on direct evidence to prove identity as the jury tends to be quite influenced by an emphatic positive identification. Note: be warry that eyewitness identification is subject to influence and human frailty.
Second Exception to the Rule Disallowing Opinion Evidence: Expert Opinion
Expert Opinion are admissible based on relevance, necessity, policy and qualification. The function of an expert is to provide the judge and jury with a ready-made inference which the judge and jury, due to the technical nature of the facts, are unable to formulate. If, on the proven facts, a judge or jury can form their own conclusion without help, then the opinion of the expert is unnecessary. Experts should not complicate the matter so as to confuse the trier of fact.
Expert evidence is usually held to a higher level of srcutiny, the degrees of which grow stronger based on the below factors before it is admissible:
- necessary to assist the trier of fact,
- exclusion under another policy rule:
See the Ultimate Issue Rule: experts cannot express an opinion on the guilt of the defendant or on the credibility of a particular witness. For example, lie detectors or polygraphs are generally inadmissible as they are used as a means of psychological intimidation. They are also seen as a perversion of the adversary system because it takes the decision of conviction away and are not an objective science).
- properly qualified expert:
The expert must be qualified and credible. Generally, notice of a use of an expert should be given in advance, and the qualifications of the expert (within a defined field) should be given to the court. The expert should be available for cross-examination on the issue/science of this testimony, as well as on his credentials (the latter possibly in a voir dire).