Copyright 2000 American Medical Association. All Rights Reserved. Applicable FARS/DFARS Restrictions Apply to Government Use.2000
ODDS RATIOS and relative risks are commonly used to express results in clinical studies. The results of cohort studies and case-control studies are best expressed as relative risks and odds ratios, respectively. The use and interpretation of these ratios are the subject of this brief review.
In a cohort study, a group of individuals exposed to an agent are compared with an appropriately selected control group of individuals who are not exposed. Both groups are observed until an event of interest occurs or for a prespecified time period. The association of exposure and outcome is expressed as the relative risk (Table 1). Assuming that a study is unbiased, the relative risk is interpreted as follows: A relative risk of 1 implies no association. If the relative risk is greater than 1, and its 95% confidence interval does not include 1, then the result implies a positive association between exposure and outcome at the 5% significance level (ie, the outcome is more likely in the exposed cohort) (Table 1, study 3). If the relative risk is less than 1, and its 95% confidence interval does not include 1, then the result implies a negative association between exposure and outcome at the 5% significance level (ie, the outcome is less likely in the exposed cohort).1,2 If the 95% confidence interval includes the relative risk of 1, then an association of exposure and outcome is not proved by the study at the 5% significance level Table 1, studies 1 and 2).
Bigby M. Odds Ratios and Relative Risks. Arch Dermatol. 2000;136(6):770-772. doi:10.1001/archderm.136.6.770