TY - JOUR

T1 - Effect of imbalance and intracluster correlation coefficient in cluster randomization trials with binary outcomes when the available number of clusters is fixed in advance

AU - Ahn, Chul

AU - Hu, Fan

AU - Skinner, Celette Sugg

AU - Ahn, Daniel

N1 - Funding Information:
This work was supported in part by NIH grants UL1 RR024982, R01 CA122330, and R01 HL087768.

PY - 2009/7

Y1 - 2009/7

N2 - In some cluster randomization trials, the number of clusters cannot exceed a specified maximum value due to cost constraints or other practical reasons. Donner and Klar [Donner A, and Klar N. Design and analysis of cluster randomization trials in health research. Oxford University Press 2000] provided the sample size formula for the number of subjects required per cluster when the number of clusters cannot exceed a specified maximum value. The sample size formula of Donner and Klar assumes that the number of subjects is the same in each cluster. In practical situations, the number of subjects may be different among clusters. We conducted simulation studies to investigate the effect of the cluster size variability (κ) and the intracluster correlation coefficient (ρ) on the power of the study in which the number of available clusters is fixed in advance. For the balanced case (κ = 1.0), i.e., equal cluster size among clusters, the sample size formula yielded empirical powers close to the nominal level even when the number of available clusters per group (k*) is as small as 10. The sample size formula yielded empirical powers close to the nominal level when the number of available clusters per group (k*) is at least 20 and the imbalance parameter (κ) is at least 0.8. Empirical powers were close to the nominal level when (ρ ≤ 0.02, κ ≥ 0.8, and k* = 10) or (ρ ≤ 0.02, κ = 0.8, and k* = 20).

AB - In some cluster randomization trials, the number of clusters cannot exceed a specified maximum value due to cost constraints or other practical reasons. Donner and Klar [Donner A, and Klar N. Design and analysis of cluster randomization trials in health research. Oxford University Press 2000] provided the sample size formula for the number of subjects required per cluster when the number of clusters cannot exceed a specified maximum value. The sample size formula of Donner and Klar assumes that the number of subjects is the same in each cluster. In practical situations, the number of subjects may be different among clusters. We conducted simulation studies to investigate the effect of the cluster size variability (κ) and the intracluster correlation coefficient (ρ) on the power of the study in which the number of available clusters is fixed in advance. For the balanced case (κ = 1.0), i.e., equal cluster size among clusters, the sample size formula yielded empirical powers close to the nominal level even when the number of available clusters per group (k*) is as small as 10. The sample size formula yielded empirical powers close to the nominal level when the number of available clusters per group (k*) is at least 20 and the imbalance parameter (κ) is at least 0.8. Empirical powers were close to the nominal level when (ρ ≤ 0.02, κ ≥ 0.8, and k* = 10) or (ρ ≤ 0.02, κ = 0.8, and k* = 20).

KW - Binary outcomes

KW - Intracluster correlation

KW - Varying cluster size

KW - sample size

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U2 - 10.1016/j.cct.2009.03.007

DO - 10.1016/j.cct.2009.03.007

M3 - Article

C2 - 19348965

AN - SCOPUS:67349122767

VL - 30

SP - 317

EP - 320

JO - Contemporary Clinical Trials

JF - Contemporary Clinical Trials

SN - 1551-7144

IS - 4

ER -