In a case-control study of oral cancer, 60 of 100 cases chewed tobacco compared with 20 of 100 controls. The odds ratio for tobacco chewing and oral cancer is:
- A 3.0
- B 6.0 ✓
- C 9.0
- D 12.0
Correct answer: B. 6.0
Explanation
Odds ratio = (ad × bc) / (bc of cross product): (60 × 80) / (40 × 20) = 4800 / 800 = 6.0. Chewing tobacco carries six times the odds of oral cancer in this sample. Option A (3.0) comes from incorrectly using only the exposure ratio between cases and controls (60/20), ignoring the unexposed cells. In a rare-disease setting this odds ratio approximates the relative risk.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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