Risk Ratio (Relative Risk)
- Risk is the average probability that an event (disease) will occur given particular exposure (Last, p. 159).
- 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
- The risk ratio is the ratio of two risks, usually exposed/unexposed: RR = [a/a+b]/[c/c+d]
- 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.