Can expert opinions based on assumed facts be admissible at trial?

Chief Justice McEachern warned about the admissibility of expert opinions based on assumed facts in Mazur v. Moody, [1987] B.C.J. No. 1027. At page 243 he wrote: The assumptions made by the chartered accountant in order to arrive at his conclusions presuppose the jury making all possible findings of fact in favour of the Plaintiff and usurp the function of the jury in that behalf. It is undoubtedly true that the jury would be entitled to make different findings of fact from those assumed by the chartered accountant and a trial judge would have to be at great pains to instruct the jury appropriately. The fact remains, however, that this alleged loss, which would be very difficult for counsel responsibly to argue, has been placed before the jury not as a submission of counsel which could hardly be made with a straight face, but as evidence.