[Odds ratio: theoretical and practical issues].

Odds Ratio (OR) is an effect measure frequently used to communicate results of health research. Mathematically, OR is the quotient between two odds, being odds an alternative way to express possibility of occurrence of an outcome or presence of an exposition. From a methodological perspective, OR can be calculated from prospective, retrospective and cross-sectional designs, and under certain conditions it can replace the Relative Risk. Based on a series of questions and examples, this article explains theoretical and methodological grounds underlying the concept of OR, in order to facilitate its interpretation for clinicians and researchers.