在流行病统计学中，相对风险（relative risk）是指在暴露在某条件下，一个（产生疾病的）事件的发生风险。相对风险概念即是指一暴露群体与未暴露群体发生某事件的比值。 若将暴露群体写成pE、未暴露群体写成pN，则算式如下

举例来说，在吸烟者中发生癌症的机率是20%，而在非吸烟者中发生癌症的机率是1%。我们将这样的描述做成一个2x2的表格如下:

如表格，a=20，b=80，c=1，d=99，所以依照公式:

因此我们得知，抽烟者得癌症的相对风险是非抽烟者的20倍。换言之，抽烟者得癌症的机率是非抽烟者的20倍。

In statistics and epidemiology, relative risk or risk ratio (RR) is the ratio of the probability of an event occurring (for example, developing a disease, being injured) in an exposed group to the probability of the event occurring in a comparison, non-exposed group. Relative risk includes two important features: (i) a comparison of risk between two "exposures" puts risks in context, and (ii) "exposure" is ensured by having proper denominators for each group representing the exposure