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Table 3 Required sample size and empirical Type I error and power when using corrected variance inflation factors with an a priori hypothesized Pareto imbalance in cluster size – Effect size = 0.25

From: Planning a cluster randomized trial with unequal cluster sizes: practical issues involving continuous outcomes

   No correction Equal weights Cluster size weights1 Minimum variance weights
Intraclass correlation coefficient (ρ) Number of clusters in each arm (g) Sample size Empirical probabilities Sample size Empirical probabilities Sample size Empirical probabilities Sample size Empirical probabilities
    Type I error Power   Type I error Power   Type I error Power   Type I error Power
0.005 5 485 0.0664 0.6432 1569 0.1028 0.8606 - - - 1037 0.0948 0.7992
  10 326 0.0566 0.6968 1057 0.0784 0.9386 515 0.0704 0.8106 464 0.0704 0.7806
  20 282 0.0486 0.7258 917 0.0624 0.9770 336 0.0450 0.7934 331 0.0458 0.7850
  40 265 0.0466 0.7572 861 0.0512 0.9918 287 0.0424 0.7842 286 0.0474 0.7706
0.02 10 629 0.0904 0.6236 2043 0.0638 0.8196 - - - 1731 0.0624 0.7968
  20 353 0.0660 0.6546 1147 0.0614 0.8924 1852 0.0576 0.9432 677 0.0752 0.7976
  40 290 0.0556 0.7008 942 0.0582 0.9486 435 0.0558 0.8092 401 0.0514 0.7960
0.05 20 743 0.0562 0.6256 2414 0.0564 0.8140 - - - 2165 0.0480 0.7976
  40 361 0.0604 0.6242 1173 0.0500 0.8598 - - - 770 0.0550 0.8048
0.10 40 652 0.0546 0.6006 2116 0.0542 0.8090 - - - 1881 0.0500 0.8036
  1. Sample size calculations were performed considering type I and type II error rates fixed at 0.05 and 0.20, respectively.
  2. 1 In some cases, 80% power was not reachable
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BMC Medical Research Methodology

ISSN: 1471-2288

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