Evidence Based Medicine Calculations

EBM Calculations Table.PNG

Relative Risk (RR) = EER/CER = (% disease in untreated patients)/(% disease in treated patients) = 0.58

  • For every 100 untreated patients, 58 will experience MI

  • For every 100 treated patients, only 33 will experience MI

  • Therefore, the risk of MI in a treated patient is only 57% of the risk for an untreated patient

Absolute risk reduction = |CER - EER| = |(% disease in untreated patients) - (% disease in treated patients)| = |0.57 - 0.33| = 0.24

  • ARR = difference between event rates in the control and treatment groups

  • If there were 100 people in the treatment group and 100 people in the control group, there would be 24 fewer MIs in the treatment group as compared to the control

Number needed to treat = 1/ARR = 1/0.24 = 4.16

  • NNT: For every 4 patients who receive the treatment, one MI will be prevented

  • If the treatment is found to increase disease rate, this calculation becomes the “number needed to harm”