Calculate Number Needed to Treat from event rates.
NNT = 1 / ARR (Absolute Risk Reduction). ARR = CER − EER. NNT tells you how many patients need to be treated for one additional patient to benefit. Lower NNT = more effective treatment. If EER > CER, the result becomes NNH (Number Needed to Harm). RRR = (CER − EER) / CER × 100%.