Basic statistic (3)

If a drug reduces the incidence of stroke from 10% to 5%, 

Relative risk reduction (RRR) is 50%

Absolute risk reduction (ARR) is 5%

OR

If the absolute risk of stroke is 10%,

the relative risk in treated group is 0.5,

then the absolute risk in treated group is 10 X 0.5 = 5%

the absolute risk reduction is 10% - 5% = 5%

Number needed to treat (NNT) is defined as number needed to treat to prevent 1 death

= 100% / ARR 

= 100% / 5%

= 20