Ratio of incidence of a disease among the exposed to the incidence among non-exposed is:
Ref. Foundations of Community medicine. Page. 596
 
Measures of effect

Measures of effect are used in epidemiological studies to assess the strength of an association between a putative risk factor and the subsequent occurrence of disease.
This requires that the incidence of disease in a group of persons exposed to a potential risk factor is compared with the incidence in a group of persons not exposed to the potential risk factor.
This comparison can be summarized by calculating either the ratio of measures of disease frequency for the two groups or the difference between the two, and reflects the increase in frequency of disease in one population compared with another, treated as baseline.

 
Three main measures of effect
Odds ratio
Relative risk
Absolute risk
Attributable risk