How is odds ratio different from relative risk

Chandraraju, Some people do use the probability ratio, aka the relative risk. Thank you. Thank you for posting. Leave a Reply Cancel reply Your email address will not be published.

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Concept of odds ratio. The OR is a comparison of the odds of an event after exposure to a risk factor with the odds of that event in a control or reference situation.

The OR is estimated as the odds of an event in the exposure group divided by the odds of that event in the control or reference group; the result is expressed as a ratio to denominator 1. Expressed in plain English, the odds of developing sexual dysfunction with venlafaxine relative to placebo are 2.

Consider a case-control study in which outpatients with depression were cross-sectionally screened for the presence of sexual dysfunction. For each patient who had sexual dysfunction, 6 age- and sex-matched depressed controls without sexual dysfunction were identified.

The results are presented in Table 2. Now, it is possible that only a few patients were prescribed antidepressants, and that the outpatient department was crowded with those who experienced antidepressant adverse effects such as sexual dysfunction. It is also possible that a lot of patients were prescribed antidepressants, that very few patients experienced adverse effects of any kind, and that the patients were doing well and so stayed away.

Therefore, without knowing how many people in total were actually prescribed antidepressants, there is no way of calculating the absolute risk of antidepressant-related sexual dysfunction. Hence, there is no way of calculating the RR using the data in Table 2. However, the OR can be calculated and, because the data are exactly the same as in Table 1 , the calculations and the result are also exactly the same. ORs are also obtained in logistic regression analyses in which the value of the crude or univariate OR for a risk variable is adjusted for the presence of measured confounders.

The adjusted OR presents a more accurate picture of the unique contribution of the risk variable to the outcome of interest. The RR and the OR should always be examined in the context of the absolute risk.

Omphalocele is rare in the population, and so, in this situation, the OR and the RR would probably be similar. If the risk of omphalocele in the general population is 0. At the individual patient level, 0. Therefore, when the absolute risk is low, even a large increase in the RR or OR may not be clinically significant. In contrast with the example explained above, if the absolute risk is high, then even a small increase in the RR or OR could be clinically important.

In a nutshell, doubling of risk is of little clinical importance when the absolute risk is low; thus, for an individual patient, an increase in risk from 0. Similar considerations apply to, for example, halving of risk. Clinicians should therefore consider the absolute risk, draw upon their knowledge of the field, consider the clinical importance of the event, and exercise their judgment when interpreting the importance of an RR or an OR when they manage the individual patient.

RRs and ORs should therefore be interpreted in the context of time as well as absolute risk. The risk of an illness is the chance, or the likelihood, that a person will develop that illness. A conditional risk, in contrast, is the chance, or the likelihood, that a person who fulfills some condition will develop an illness.

Streiner DL. Risky business: making sense of estimates of risk. Can J Psychiatry. Statistics notes: the odds ratio. PubMed doi Kaelin MA, Bayona M. Open Menu. Odds Ratio to Risk Ratio Conversion. Updated July 1, Accessed November 14, All rights reserved. Then wait and see how many in each group had strokes.

But in the fast-moving Covid environment, a leisurely, well-designed study like that is not feasible. So we take a reverse approach: look at the patients who did have strokes, and see if their treatment protocols were less likely to include blood thinners. But… less likely than what? So now we have two groups; one with all-strokes and the other, the case-control group, with strokes and no strokes.