The CAPRIE study was a randomised clinical trial published in Lancet in 1996 to assess the potential benefit of clopidogrel, compared with aspirin, in reducing the risk of ischemic stroke, myocardial infarction, or vascular death in patients with recent ischemic stroke, recent myocardial infarction, or peripheral arterial disease. The study's primary endpoint cluster of incidence of ischemic stroke, myocardial infarction, and or vascular death occurred in 5.32% of clopidogrel patients per year and 5.83% of aspirin patients per year (p=0.043). Assuming this p-value is statistically significant, what is the number needed to treat in order to prevent the primary endpoint by using clopidogrel compared to aspirin?

Answer with rationale:

The correct answer is 196. In order to get the number needed to treat (NNT), you first have to subtract the difference between the two absolute event rates given (5.83% for clopidogrel-5.32% for aspirin) to give 0.51%. The NNT is calculated then by 1/Absolute risk reduction (ARR). The ARR  must be converted from a % to decimal, so this would be 1/0.0051 which gives 196. 

Reference:

1. CAPRIE Steering Committee. A randomised, blinded, trial of clopidogrel versus aspirin in patients at risk of ischemic events (CAPRIE). Lancet 1996;348:1329-39.