1. What is Relative Risk?

Relative Risk (RR) is a measure of the strength of association between an exposure and an outcome. It compares the probability of an event occurring in the treatment group versus the control group.

2. How to Calculate Relative Risk

The formula for Relative Risk is:

Where:

• \( event_{treatment} \) — Number of events in treatment group
• \( n_{treatment} \) — Total number in treatment group
• \( event_{control} \) — Number of events in control group
• \( n_{control} \) — Total number in control group

Example: If 20 out of 100 treated patients have an event (20%) and 10 out of 100 control patients have the event (10%), RR = (20/100)/(10/100) = 2.0

3. Interpreting Relative Risk

Interpretation:

• RR = 1: No difference in risk between groups
• RR > 1: Increased risk in treatment group
• RR < 1: Decreased risk in treatment group (protective effect)

4. When to Use Relative Risk

Use cases: Relative Risk is commonly used in cohort studies and randomized controlled trials to compare risk between exposed and unexposed groups.

5. Frequently Asked Questions (FAQ)

Q1: What's the difference between RR and OR?
A: Relative Risk compares probabilities directly, while Odds Ratio compares odds. RR is more intuitive but can't be used in case-control studies.

Q2: When is RR preferred over OR?
A: RR is preferred when the outcome is common (>10%) as OR can overestimate the effect size in these cases.

Q3: How do I interpret RR=0.5?
A: RR=0.5 means the treatment group has half the risk of the outcome compared to the control group.

Q4: What are limitations of RR?
A: RR doesn't convey absolute risk differences and can be misleading when baseline risks are very small.

Q5: Should I report confidence intervals with RR?
A: Yes, always report 95% confidence intervals to show the precision of your RR estimate.