[¶ 121] Procedural advantages of a proof of assumption rule. A construction of s 79 which does not require establishment at the time of tender that there either has been, or will be, evidence admitted capable of proving the assumed facts permits more expert opinion evidence to be received. It permits postponement of the difficulties by seeking to solve them as questions of weight at a later time -- even as late as the end of the trial. 167 But increasing the amount of this type of evidence is not necessarily valuable. It may be unfair to the opposing party. It is indecisive. Its indecisiveness inflicts uncertainties on the parties. The additional evidence received may have a cloud over it for the rest of the trial.
[¶ 122] In contrast, a proof of assumption rule diminishes the risk of clouds. It encourages early and decisive rulings. Early and decisive rulings are important, both for the party opposing tender and for the tendering party.
[¶ 123] From the point of view of the party opposing tender, it is vitally important to know what evidence is or is not in, and how much utility expert opinion evidence is likely to have. That knowledge affects decisions about cross-examining the witnesses called by the tendering party; decisions by defendants whether or not to submit that there is no case to answer; decisions whether or not to call particular categories of evidence; and, if rulings are delayed until after the close of the trial, decisions about what is to be said in address. A practice of deciding whether a proof of assumption rule has been complied with at the time when expert opinion evidence is tendered avoids a dilemma for cross-examiners. One horn of the dilemma is that to cross-examine a witness about expert evidence which may later be rejected or treated as useless carries the risk of giving it a foothold in the record which it lacked at the time of the tender. The other horn of the dilemma is that, if the opposing party avoids that danger by not cross-examining on the expert evidence, there is a risk that it will be accepted despite its feebleness. It is a dilemma which cross-examiners should not have to face.
[¶ 124] From the point of view of the tendering party, it is desirable that the admissibility of expert opinion evidence tendered by that party be clear by the moment when the case for that party closes. It is undesirable that expert opinion evidence admitted in that party's case should later be held -- perhaps as late as the time of judgment -- to be subject to such doubts about its weight that it lacks utility. It is undesirable that its admissibility be in suspense until a time after the tendering party's case has closed. If the admissibility of expert opinion evidence which is tendered and conditionally admitted is not finally ruled on until after the case for the tendering party is closed, and the evidence is then rejected, or its weight has become so questionable that it is useless, the tendering party may have lost an opportunity to repair the position before its case closed, either by calling further witnesses or tendering further documents, or by recalling witnesses who had already been in the box. The capacity of tendering parties who are the prosecution or the plaintiff to reopen their cases rests on a discretion in the court which may not be favourably exercised; their capacity to tender evidence in reply is constricted by fairly strict rules, particularly in criminal cases.
[¶ 125] Jury trials. There are yet further difficulties in relation to jury trials. If evidence is rejected when tendered, the jurors are not confused by it, for they will ordinarily be absent during the debate about the tender: s 189(4) of the Act. If circumstances change and evidence once rejected becomes admissible, it can be re-tendered successfully. Again there is no risk of jury confusion. However, considerable confusion can flow where, although opinion evidence is admitted conditionally, later it becomes apparent that the condition is not satisfied. The evidence must be removed from the record, otherwise there would be no difference between conditional and unconditional admission. The same problems arise where opinion evidence is admitted, not on any formal condition, but simply in the expectation that at some time after the tender of the opinion evidence, witnesses will be called to establish the factual assumptions on which the opinion was pronounced, but that expectation is disappointed. In either event the jury will have heard evidence which is inadmissible. Should it be struck out? Should it be withdrawn from the jury? Should the jury be directed that the issue to which the expert's evidence was directed no longer arises? Should the jury be told not to consider the expert's evidence? Should the jury be told to disregard the expert's evidence on the ground that the factual basis has not been proved? 168 (168. See Wigmore, Evidence in Trials at Common Law, Tillers rev (1983), Vol 1 at 702-731 [Sections] 14 - 14.1 and 847-855 [Section] 19.) All these courses are possible. Each course is less attractive than a regime having a proof of assumption rule and a practice of rejecting the tender until it has been satisfied.
[¶ 126] And what is to be done with any evidence that was called in relation to that conditionally admitted but inadmissible evidence, whether it was elicited by the cross-examination of the party opposing tender or tendered by that party in its own case? That problem is reduced if decisive rulings about compliance with a proof of assumption rule are made.
[¶ 127] Conclusion. A construction of s 79 which holds that there is no proof of assumption rule in relation to s 79 tenders is difficult to reconcile with the practical exigencies pursuant to which parties conduct their cases. It is necessary for trials to be conducted in a businesslike and efficient way. That is a matter of context pointing to the view that there is a proof of assumption rule with which those tendering expert opinion evidence must comply by reason of ss 55, 56 and 79 read against the background of the common law.