Risk Difference (Attributable Risk)

The risk difference (RD) measures the absolute difference in risk between exposed and unexposed groups:

$$RD = p_1 - p_2 = \frac{a}{a+b} - \frac{c}{c+d}$$

Where:

• $p_1 = a/(a+b)$ = risk in exposed group
• $p_2 = c/(c+d)$ = risk in unexposed group

95% Confidence Interval:

$$CI_{95\%} = RD \pm 1.96 \times SE_{RD}$$

Where:

$$SE_{RD} = \sqrt{\frac{p_1(1-p_1)}{n_1} + \frac{p_2(1-p_2)}{n_2}}$$

• $n_1 = a+b$ = total in exposed group
• $n_2 = c+d$ = total in unexposed group