Understanding Relative Risk of 0.5
A relative risk (RR) of 0.5 means the intervention reduces the risk of the outcome by 50% compared to the control group—in other words, the event occurs half as often in the treated group. 1
Mathematical Interpretation
- A relative risk is calculated as the ratio of the probability of an outcome occurring in the intervention group divided by the probability in the control group 1
- An RR of 0.5 indicates that individuals receiving the intervention have half the risk (50% lower risk) of experiencing the outcome compared to those in the control group 1
- Any RR less than 1.0 indicates the intervention reduces risk, while an RR greater than 1.0 indicates increased risk, and an RR of exactly 1.0 indicates no difference between groups 1
Clinical Examples from Guidelines
- In suicide prevention interventions, an RR of 0.570 (95% CI 0.408-0.795) meant that safety planning reduced suicidal behavior by 43%, with a number needed to treat of 16 1
- In cardiovascular studies, an RR of 0.69 (95% CI 0.51-0.92) for mortality with PCI versus fibrinolytic therapy represented a 31% reduction in death 1
- For breastfeeding and childhood obesity, an RR of 0.78 meant breastfed children had 22% lower risk of obesity compared to never-breastfed children 1
Absolute vs. Relative Risk Reduction
- The absolute risk reduction depends on the baseline risk in the population—the same RR of 0.5 produces different absolute benefits depending on how common the outcome is 2
- For example, if the baseline risk is 20%, an RR of 0.5 reduces absolute risk to 10% (10% absolute reduction); if baseline risk is 2%, it reduces to 1% (only 1% absolute reduction) 2
- This is why confidence intervals and baseline risk context are essential—an RR of 0.5 with wide confidence intervals crossing 1.0 would not be statistically significant 1, 3
Statistical Significance Considerations
- The RR point estimate must be accompanied by 95% confidence intervals to assess both statistical significance and precision 3
- If the confidence interval for an RR of 0.5 does not cross 1.0 (e.g., 95% CI 0.35-0.75), the result is statistically significant 1, 3
- A p-value < 0.05 alongside an RR of 0.5 indicates less than 5% probability the observed reduction occurred by chance alone 3
Important Caveats
- The RR can shift toward the null value (1.0) as outcome prevalence increases, even when the true effect magnitude remains constant—this is a mathematical property of the ratio 4
- When outcomes are common (≥10% in the unexposed group), the RR may not accurately reflect the true effect, and odds ratios may be more appropriate 5
- In rare outcome scenarios with zero events in one group, specialized statistical methods are needed to calculate valid confidence intervals for RR 6
- The clinical importance of an RR of 0.5 depends heavily on the severity of the outcome, baseline risk, and number needed to treat—a 50% reduction in mortality is more clinically meaningful than a 50% reduction in mild side effects 2