Absolute Risk Reduction and Calculating a NNT– is the difference between the control event rate and the experimental event rate.
Moderate exacerebation; ARR= 0.329 – 0.301 = 0.028 (2.8%)
The NNT is the inverse of the ARR – 1/ARR = 1/0.028 = 36
Therefore, you will need to treat 36 patients with tiotropium to prevent 1 moderate exacerbation when compared to salmeterol.