Home
Back
ARR Approximation Formula:
| From: | To: |
Absolute Risk Reduction (ARR) is the difference in event rates between control and treatment groups in clinical trials. It provides a direct measure of treatment effect size and is more clinically meaningful than relative measures like odds ratio.
The calculator uses the ARR approximation formula:
Where:
Explanation: This formula approximates ARR from OR when the actual event rates are not available, providing a useful estimate for clinical decision-making.
Details: ARR is crucial for understanding the actual clinical impact of treatments, calculating Number Needed to Treat (NNT), and making informed treatment decisions based on absolute rather than relative benefits.
Tips: Enter odds ratio (any positive value) and control event rate as a proportion between 0 and 1. For example, 15% event rate should be entered as 0.15.
Q1: Why Use This Approximation Instead Of Exact Calculation?
A: This approximation is useful when only odds ratio is reported in studies, allowing estimation of ARR without access to raw event rate data.
Q2: How Accurate Is This Approximation?
A: The approximation works best when event rates are low to moderate (less than 30%). Accuracy decreases with very high event rates.
Q3: What Is The Relationship Between ARR And NNT?
A: Number Needed to Treat (NNT) is calculated as 1/ARR. For example, ARR of 0.1 corresponds to NNT of 10.
Q4: When Should I Use ARR Instead Of Relative Risk Reduction?
A: ARR provides the actual difference in risk, making it more clinically relevant for understanding treatment impact, especially when baseline risks vary.
Q5: Can This Formula Be Used For Harm Outcomes?
A: Yes, when OR > 1 indicates harm, the formula calculates Absolute Risk Increase (ARI), which is the negative of ARR.