比（ratio）或比值，在数学上是一对孪生术语，比是两个非零“数”或非零“同类量”之间的比较关系，另可用以表示部分占全体的相对关系；两数或量前后或上下并置，分别称为前项与后项，记作 a : b {\displaystyle a:b} ，读作 a比b，如右图之 4:3。

An odds ratio is a statistic that quantifies the strength of the association between two events, A and B. The odds ratio is defined as the ratio of the odds of A in the presence of B and the odds of A in the absence of B, or equivalently, the ratio of the odds of B in the presence of A and the odds of B in the absence of A. Two events are independent if and only if the OR equals 1, i.e., the odds of one event are the same in either the presence or absence of the other event.