NNT (Number Needed to Treat): the number of patients who need to be treated to prevent 1 adverse outcome. Specifies the treatment, its duration, and the adverse outcome being prevented. Reported as a whole number, calculated as 1/ARR, rounded to the next highest whole number, and accompanied by its 95% confidence interval (CI). 1/ARR = 1/6.8% = 14.7, rounded to 15, with a 95% CI from 9 to 35. Readers can convert this to an NNT for a specific patient by estimating that patient's susceptibility relative to the average control patient in the trial report, expressing it as a decimal fraction, F, and then dividing the reported NNT by F. If a reader's patient is judged to be half as susceptible as the average control patient in the example, F = 0.5 and NNT/F = 15/0.5 = 30. 

From http://www.shef.ac.uk/~scharr/ir/nnt.html