In personal injury cases involving aggravation of pre-existing conditions, jury directions must clearly distinguish between two separate exercises: (1) identifying the extent of the aggravation injury through a before-and-after enquiry, and (2) discounting damages for the pre-existing risk of further injury (vicissitudes). Directions that conflate these exercises by using the language of 'apportionment' of the plaintiff's global condition risk misleading the jury. Expert evidence expressed in terms of apportionment of current impairment between pre-existing and compensable causes requires careful direction as to its proper use. Counsel addressing a jury should not express personal opinions about opposing counsel's damages figures or refer to awards in other cases.
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