In Bleta v. R., 1964 CanLII 14 (SCC),  S.C.R. 561 at 564, Ritchie J. discussed the inappropriateness of an expert basing its opinion on facts that are in dispute and when the credibility of certain evidence is contested: … it is because the opinion of an expert witness on such a question can serve only to confuse the issue unless the proven facts upon which it is based have been clearly indicated to the jury that the practice has grown up of requiring counsel, when seeking such an opinion, to state those facts in the form of a hypothetical question. In cases where the expert has been present throughout the trial and there is conflict between the witnesses, it is obviously unsatisfactory to ask him to express an opinion based upon the evidence which he has heard because the answer to such a question involves the expert in having to resolve the conflict in accordance with his own view of the credibility of the witnesses and the jury has no way of knowing upon what evidence he based his opinion. Where, however, there is no conflict in the evidence, the same difficulty does not necessarily arise and different considerations may therefore apply.
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