Risk Ratio (Relative Risk)

  1. Risk is the average probability that an event (disease) will occur given particular exposure (Last, p. 159).
  2. Using the 2×2 table (Please see Aschengrau & Seage, Chapter 3):
Disease + Disease - Total
Exposure + a b a+b
Exposure - c d c+d
Total a+c b+d a+b+c+d

Probability of Disease in the Exposed: a/a+b
Probability of Disease in the Unexposed: c/c+d

  1. The risk ratio is the ratio of two risks, usually exposed/unexposed: RR = [a/a+b]/[c/c+d]
  2. Interpretation: During time t, the risk of developing disease was (n times higher/n times less likely) in the exposed persons compared with non-exposed persons in this population.