What is the difference between Absolute Risk Reduction (ARR) and Relative Risk Reduction (RRR)? This is a common question from students and clinicians. Understanding these concepts is crucial for interpreting research findings, especially in clinical and public health settings.
Absolute Risk Reduction (ARR) refers to the difference in outcomes between a control group and a treated group in a clinical trial or an public health study.
Formula: ARR = CER – EER
Where: CER is the Control Event Rate (rate of event in the control group) and EER is the Experimental Event Rate (rate of event in the experimental group).
Example: Imagine a trial in which 10% of patients in the control group have an adverse event, and only 5% in the treatment group experience the same.
ARR = 10% – 5% = 5%
This means that the drug reduces the absolute risk of an adverse event by 5%. In total, 20 people need to be treated to prevent one event (the Number Needed to Treat, NNT).
Relative Risk Reduction (RRR) is the proportional reduction in outcomes between the treated and untreated groups. It’s a way to contextualize the effectiveness of a treatment by considering the baseline risk.
Formula: RRR = {(CER – EER)}{CER} \times 100
Example: Continuing with the same drug trial, RRR = {(10% – 5%)}{10%} \times 100 = 50%
Interpretation: The drug reduces the relative risk of an adverse event by 50% compared to the control group.
Key Differences between ARR and RRR
- Context: ARR gives you the actual change in risk, which is straightforward and easily interpretable. RRR puts this change in the context of the baseline risk, making the treatment appear seem more effective than it may actually be.
- Impact: ARR is more useful for understanding the individual benefit of an intervention, while RRR is often more impressive for public health interventions where a small absolute change can have a large impact when scaled up.
- Communication: RRR is often used in marketing or in media because it tends to produce a larger, more eye-catching number. However, this can be misleading if not used with the ARR, which provides a more direct measure of an intervention’s effect.
- Clinical Relevance: Knowing both ARR and RRR can aid in shared decision-making between clinicians and patients. While RRR can show the effectiveness of a treatment, ARR can guide on how much benefit an individual patient can expect.
By understanding both Absolute Risk Reduction and Relative Risk Reduction, clinicians and public health specialists can better interpret the data from clinical, public and epidemiological studies, and subsequently make more informed decisions about treatment options and public health interventions.