Two radiologists independently classify 100 chest X-rays as normal or abnormal. They agree on 85 X-rays. The observed agreement is 85%, but the expected agreement by chance alone is 50%. The kappa statistic is:
- A 0.35
- B 0.50
- C 0.70 ✓
- D 0.85
Correct answer: C. 0.70
Explanation
Kappa equals (observed agreement minus expected agreement) divided by (1 minus expected agreement). Here, kappa equals (0.85 minus 0.50) divided by (1 minus 0.50), which is 0.35 divided by 0.50, giving 0.70. Kappa measures agreement beyond chance. Option A is the numerator alone. Option B is the expected agreement. Option D is the raw observed agreement.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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