Relative Risk (Risk Ratio)

The relative risk (RR) compares the risk of an outcome in an exposed group to the risk in an unexposed group:

$$RR = \frac{a/(a+b)}{c/(c+d)} = \frac{p_1}{p_2}$$

Where:

• a = exposed with disease
• b = exposed without disease
• c = unexposed with disease
• d = unexposed without disease
• $p_1 = a/(a+b)$ = risk in exposed group
• $p_2 = c/(c+d)$ = risk in unexposed group

95% Confidence Interval:

$$CI_{95\%} = RR \times e^{\pm 1.96 \times SE_{\ln(RR)}}$$

Where:

$$SE_{\ln(RR)} = \sqrt{\frac{b}{a(a+b)} + \frac{d}{c(c+d)}}$$