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Risk Ratios and Odds Ratios in Medicine

The odds ratio is a certain measure of the strength of association between an outcome and exposure of an event. It means that this concept represents the likelihood of an occurrence of a particular result of interest given a specific risk factor compared to the odds of the given outcome in case the exposure is absent (“Odds ratio,” n.d.). According to Szumilas (2010), “odds ratios are most commonly used in case-control studies; however, they can also be used in cross-sectional and cohort study designs as well (with some modifications and/or assumptions)” (p. 227). What is more, researchers note that “the odds ratio can also be used to determine whether a particular exposure is a risk factor for a particular outcome, and to compare the magnitude of various risk factors for that outcome” (Szumilas, 2010, p. 227). In medicine, odds ratios may be used to compare the odds of occurrence of a particular disorder or disease provided the exposure to medical history aspect or health characteristics.

Risk ratio is a rather intuitive concept compared to odds ratio. It is a specific measure that compares the probabilities for two particular groups (“Risk ratios and rate ratios (relative risk),” 2018). In other words, it is the concept defining the risk of one concrete outcome over the possibility of another. It is not always recommended to use risk ratios, and many researchers prefer avoiding this measure because, compared to odds ratios, it is not meaningful at any prevalence (“Risk ratios and rate ratios (relative risk),” 2018). However, it is still applied to clinical trials, longitudinal cohort studies, to compare conditions before and after treatment, and in case of incidence being more than ten percent.

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Of course, there are certain differences between odds ratios and risk ratios. To begin with, the primary reason for distinguishing between these two unique concepts is that the former is a ratio of two odds, while the latter is a ratio of two probabilities, and these are entirely different terms (“Risk ratios and rate ratios (relative risk),” 2018). What is more, risk ratios, compared to odds ones, may be interpreted in a straightforward way. For instance, a risk ratio of five means that there is a fivefold increase in the outcome risk, but an odds ratio of five is not interpreted the same. On the contrary, it means that the odds of an outcome of interest are fivefold higher. Further, with odds ratios, there is generally a slight overestimate of the outcome’s effect (“Odds ratio,” n.d.). Though for rare outcomes, both measures may be similar and show less than ten percent, when outcomes exceed ten percent, the risk ratio becomes overestimated.

There are particular advantages and disadvantages of both measures. To begin with, the drawbacks of odds ratios involve the fact that this concept is challenging to interpret directly. That is why this measure is often comprehended as being equivalent to the relative risk (Szumilas, 2010). Nevertheless, in case the initial risk is high, the odds ratio does not approximate appropriately to the relative risk. Therefore, this measure and its interpretation may mislead. The advantage of this concept is that in some cases, it is the only measure allowed by some designs, and covariate adjustment for it is easier (“Odds ratio,” n.d.). As for risk ratios, issues with them include the possibility of this measure producing risk estimates that are higher than one hundred percent if the prevalence is increased (“Risk ratios and rate ratios (relative risk),” 2018). Nevertheless, their interpretation does not overestimate the outcome’s effect, and it is the advantage of risk ratios.


Odds ratio. (n.d.). PsychSceneHub.

Risk ratios and rate ratios (relative risk). (2018). Measures of Association.

Szumilas M. (2010). Explaining odds ratios. Journal of the Canadian Academy of Child and Adolescent Psychiatry19(3), 227–229.