Where expert opinion evidence is admitted based on assumptions, and those assumptions are not established by the evidence at trial, the opinion is of little or no value and cannot support a conviction beyond reasonable doubt. A trial judge's ultimate conclusion must be consistent with intermediate findings — a finding that the Crown cannot exclude a reasonable hypothesis consistent with innocence is fatal to a conviction on a circumstantial case. Where both the Crown and defence experts agree on a critical factual matter (here, that brake fade occurred), the trial judge cannot reject that agreed position by resort to 'common sense'.
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