Razi Journal of Medical Sciences

Relative risk (RR) and Odds ratio (OR) are two measures of risk which has been widely used in epidemiological studies. The cohort studies are designed to directly compute a relative risk, but the situation is somewhat more complicated in the case-control design. In these studies, the odds ratio gives a valid estimate of the effect size. There is not a serious problem if you don’t use of the odds ratio as an estimator of relative risk, but the problem begins when the odds ratio used as a surrogate for relative risk while its meaning is forgotten as the ratio of tow odds. The author has tried, to describe the relationship between relative risk and odds ratio and also the right time to use each of theme for those who familiar with the basic concept of epidemiologic studies. In this paper, the concepts of probability and chance and definitions refer to relative risk and odds ratios and characteristics of each have been reported. Afterward the gap between RR and OR according to their computational formulas are described and a simple method to estimate the OR to RR ratio is expressed.