A cohort study finds an Attributable Risk (AR) of 15 per 1000 person-years and an incidence in the unexposed group of 5 per 1000 person-years. The Relative Risk is:
- A 2.0
- B 3.0
- C 4.0 ✓
- D 5.0
Correct answer: C. 4.0
Explanation
AR = Incidence in exposed (Ie) − Incidence in unexposed (Iu). So Ie = AR + Iu = 15 + 5 = 20 per 1000. RR = Ie / Iu = 20 / 5 = 4.0. AR tells the absolute excess risk attributable to exposure, while RR is the ratio comparing incidence in exposed versus unexposed.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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