2-344 Expert opinion may be called to assist the court in matters of legitimacy as to whether periods of gestation could be too long or too short (a); a psychologist may give evidence of the respective personalities of defendants based on commonly-employed clinical tests (b); a medial witness may express an opinion whether wounds on a body he had not seen could have been self-inflicted, assuming the facts described by another witness who had seen the body were true (c); a policy officer with fifteen years' experience in a traffic division who had attended a course of accident investigation and attended to more than 400 fatal road accidents, can give expert evidence of his theories and conclusions on an accident (d). Voice identification is a field where expert opinion is admissible, even though it is not an exact science (e). An expert in the production of high quality sound recordings may give evidence as to voice identity, having based his opinion on a comparative tape recording which he had prepared (f).
Evidence of a doctor, both of fact and opinion, may be admitted if -
A non-expect witness may give admissible evidence whether a person was drunk (l) (but not whether he was fit to drive), whether an estimate as to the value of damage to a window was accurate (m).
Expert opinion is necessary only when the expert can furnish the court with scientific information likely to be outside its experience; for example a psychiatrist is not necessary to say how an ordinary person who is not suffering from mental illness is likely to react to the stresses and strains of life (n); or the effects of cocaine and its various methods of ingestion (o); expert evidence on whether material is obscene is probably admissible only where a special audience is in contemplation (p).
An expert may refer to professional treatises, tables, reports etc to refresh his memory, but it is his evidence and not that material which is admissible (q). When an expert witness is asked to express his opinion on a question, the primary facts on which that opinion is based must be proved by admissible evidence given either by the expert himself or some other competent witness. However once such facts are proved, the expect witness is then entitled to draw on the work (including unpublished work) of others in his field of expertise as part of the process of arriving at his conclusion, provided he refers to that material in his evidence so that the cogency and probative value of his conclusion can be tested by reference to that material. Reliance on the work of others and reference to it in evidence does not infringe the hearsay rule in these circumstances (r).