Number needed to treat (NNT)

1. Formula: NNT = 1 / ARR
• NNT: number needed to treat
• ARR: absolute risk reduction (PLACEBO GROUP RATIO EVENT)
  ARR = AR: attributable risk = (a / a + b) - (c / c + d)
2. Classic USMLE question:
• "You are asked to evaluate the efficacy if a new drug X effect in laboratory rats. The results of a randomized control trial at 10 years are given below:
                                           Disease +          Disease -
  Treated  (drug X)                  10                      990
  No treated  (placebo)           25                      975
  
  What is the number needed to treat to prevent the disease using drug X?
• Answer
  NNT = 1 / ARR = 1 / (placebo group - treatment group)
  a / a + b = treatment group = 10 / (10 + 990) = 0.010
  c / c + d = placebo group = 25 / (25 + 975) = 0.025
  
  NNT = 1 / (0.025 - 0.010) = 1 / 0.015 = 66.6 or 67