Expert opinion evidence about what third parties would have done in hypothetical situations is inadmissible as a question of fact, not expert opinion. The assumption identification and statement of reasoning rules must be satisfied in the expert report itself and cannot be remedied after the fact by counsel's submissions. Where a party's causation hypothesis depends on a counterfactual scenario, that scenario must be pleaded with clarity sufficient to enable the opposing party to meet the case, and expert evidence directed to an unpleaded counterfactual case will be excluded.
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