In a cluster randomised trial (CRT) evaluating a community nutrition intervention, the intracluster correlation coefficient (ICC) is 0.05 with cluster size of 50. What is the Design Effect (DEFF)?
- A 1.05
- B 3.45 ✓
- C 2.45
- D 2.0
Correct answer: B. 3.45
Explanation
Design Effect (DEFF) = 1 + ICC × (cluster size − 1) = 1 + 0.05 × (50 − 1) = 1 + 0.05 × 49 = 1 + 2.45 = 3.45. The DEFF indicates that the CRT requires 3.45 times more participants than an equivalent individually randomised trial to achieve the same statistical power, because observations within clusters are correlated. Higher ICC or larger cluster size increases DEFF and thus the required sample size.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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