Which statement best describes the interpretation of relative risk reduction?

• A. It is important to understand the magnitude of the risk in the control group in order to determine the impact
• B. The p-value alone determines significance
• C. Only sample size matters for RRR
• D. RRR is independent of baseline risk

Answer: (A)

Relative risk reduction expresses how much the risk is reduced in the treatment group compared with the control, as a proportion of the control risk. The key point is that this proportion only makes sense in the context of how big the baseline risk is. The same RRR can mean very different absolute improvements depending on the control group's risk.

For example, an RRR of 50% lowers a control risk from 1% to 0.5% (an absolute reduction of 0.5 percentage points) versus from 40% to 20% (an absolute reduction of 20 percentage points). The clinical impact looks very different even though the relative reduction is the same. Therefore, understanding the control group's baseline risk is essential to gauge the real effect size.

The other ideas described don’t fit because p-values reflect statistical significance, not the magnitude of effect; RRR interpretation is not determined by sample size alone; and RRR does depend on baseline risk for its practical interpretation.