I forgot to mention that when assessing a healer you should look at the range of spells used. Before patch 3.XX dropped I was running with a healer that was consistantly underperforming. A huge part of that was her limited use of spells. Out of the heals available to a holy priest (B.Heal, F.Heal, G.Heal, Renew, CoH, ProH, ProM, and so on) she was using only four.
In addition to looking at the range of spells, an examination of their use in relation to other spells (think % of healing spells cast - for example, G.Heal making up 30% of heals cast) and in relation to their healing assignment can yield more answers.
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