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Table 5 Total number of households (children)a required for 80% and 90% power for varying control arm event rates and intra-cluster correlation coefficients

From: Levofloxacin versus placebo for the prevention of tuberculosis disease in child contacts of multidrug-resistant tuberculosis: study protocol for a phase III cluster randomised controlled trial (TB-CHAMP)

Power Contacts who develop TB disease by 48 weeks (control arm) (%) Intra-cluster correlation coefficients
0.05 0.1 0.15
80% 5 1056 (2122) 1108 (2216) 1158 (2316)
7 742 (1484) 778 (1556) 814 (1628)
10 508 (1016) 532 (1064) 556 (1112)
90% 5 1414 (2828) 1480 (2960) 1548 (3096)
7 994 (1988) 1040 (2080) 1088 (2174)
10 678 (1356) 710 (1420) 742 (1484)
  1. aAssumes 2 contacts per household, 10% loss to follow-up, two-sided 5% significance level test and a 50% risk reduction in the intervention arm
  2. The number in italics indicates the chosen trial sample size