Translations:Advanced Field Epi:Manual 1 - Disease Investigation/175/en
- a is small compared to b, and c is small compared to d
- therefore, (a+b) approximates b and (c+d) approximates d
- therefore, the RR approximates ad/bc
- so the OR estimates the RR as long as the disease is rare (say ~<10% but the approximation becomes better as the disease becomes rarer)
- When the disease gets more common the OR provides a poorer numeric approximation of the relative risk.