ARR, RRR & NNT

Exam Cram

• Absolute risk reduction

• Relative risk reduction

• Number needed to treat = 100/ ARR (%)

Risk Reduction: Absolute & Relative

Lets use the example of a cardiology drug that reduces the annual risk of MI from 6% to 2%. 

The absolute risk reduction=  6%-2% = 4% per year. 

The relative risk reduction = 6% / 2% = 3.

Number needed to treat (NNT) 

Number need to treat is the number of people required to have one successful outcome. 

Using the example above, this means for 100 individuals untreated individuals 6 will develop an MI and 100 treated individuals 2 will develop an MI. Therefore, per 100 treated individuals 4 MIs have been avoided; or 1 MI for every 25 individual treated. Therefore, in this example NNT = 25. 

NNT = 100/ ARR.

NNT = 100/ 4, = 25.