From the Guidelines
Relative risk (RR) indicates how many times more likely (or less likely) an event is to occur in one group compared to another, providing a comparative measure of the strength of association between exposures and outcomes. This measure is calculated by dividing the incidence rate in the exposed group by the incidence rate in the non-exposed group, as explained in the context of breast cancer risk reduction 1. For instance, an RR of 2.0 means people in the exposed group are twice as likely to develop the outcome than those in the non-exposed group, while an RR of 0.5 indicates the exposed group has half the risk.
Key Points to Consider
- Relative risks are often used in scientific articles because they tend to be independent of baseline risk factors and can simply summarize the proportion of disease related to a particular intervention or risk factor 1.
- However, when assessing the public health impact of an intervention or factor, relative risk can be misleading because it does not account for the baseline risk or the absolute effect size.
- Absolute risks capture the aspect of effect size that relative risks do not, but they relate to the baseline risk associated with the factor under consideration and the duration of the effect, as seen in the example of breast cancer risk reduction with tamoxifen 1.
- Understanding relative risk is essential for making informed clinical decisions, but it should be considered alongside absolute risk measures for a complete picture of clinical significance, including the number needed to treat (NNT) to achieve a specific outcome.
Clinical Application
In clinical practice, relative risk is a valuable tool for evaluating the efficacy of interventions and the impact of risk factors. However, clinicians must also consider the absolute risk and the baseline risk of the population to make informed decisions about patient care. For example, a woman with a higher baseline risk of breast cancer may benefit more from a chemopreventive intervention with a given relative risk than a woman with a lower baseline risk, as the absolute risk reduction would be greater 1.
From the Research
Definition and Interpretation of Relative Risk
- Relative risk (RR) is a measure used in epidemiology and clinical research to investigate associations between exposures and outcomes [(2,3,4,5)].
- It is defined as the ratio of the probability of an outcome in an exposed group to its probability in a non-exposed group [(3,4,5)].
- RR provides a measure of change in the likelihood of an event linked to a given intervention 3.
Advantages and Limitations of Relative Risk
- RR is considered a measure with "portability" across varying outcome prevalence, especially when the outcome is rare 3.
- However, RR can be problematic as it varies for reasons other than the magnitude of the effect, and it is dependent on baseline prevalence of an outcome 3.
- RR shifts toward its null value with increasing outcome prevalence, regardless of the strength of the association between intervention and outcome 3.
Comparison with Other Measures
- RR is often compared to the odds ratio (OR), which measures solely the effect magnitude and has no relationship to the prevalence of an outcome [(3,4,5)].
- Absolute risk reduction (ARR) is another measure that accounts for the baseline control group event rate and is a more realistic quantification of treatment effect than relative measures [(4,6)].
- Number needed to treat (NNT) estimates the therapeutic effort needed to prevent one additional adverse event and incorporates both relative risk reduction and the event rate without treatment [(4,6)].