posted on 2017-04-03, 08:40authored byCaroline A. Kristunas, Karen L. Smith, Laura J. Gray
Background
The current methodology for sample size calculation for steppedwedge
cluster randomised trials (SW-CRTs) is based on the assumption
of the clusters being of equal size. However, as is often the case
in CRTs, the clusters in SW-CRTs are likely to vary in size which in
CRTs of other designs leads to a reduction in power. The effect of an
imbalance in cluster sizes on SW-CRTs was not known, nor what an
appropriate adjustment to the sample size should be.
Trials 2016, Volume 17 Suppl 1 Page 4 of 6
Methods
We proposed three adjusted design effects (DEs) for use in the calculation
of the sample size for SW-CRTs with varying degrees of imbalance
in cluster size, based on those suggested for use in CRTs with
unequal cluster sizes. A simulation study was conducted which investigated
the effect of unequal cluster sizes on the power of SW-CRTs,
when the sample size was calculated using both the standard
method and the three proposed adjusted DEs.
Results
An imbalance in cluster size was not found to significantly affect the
power of a SW-CRT, and the proposed adjusted DEs generally resulted
in trials that were severely over-powered.
Conclusions
We recommend that the standard method of sample size calculation
for SW-CRTs be used when any imbalance in cluster size is expected
to be small. When there is likely to be a large imbalance in cluster
size it is recommended that simulations be used to determine if additional
clusters are needed.
Funding
CK is funded by a National Institute for Health Research (NIHR) Research Methods
Fellowship.
History
Citation
Trials, 2016, 17(Suppl 1):311
Author affiliation
/Organisation/COLLEGE OF MEDICINE, BIOLOGICAL SCIENCES AND PSYCHOLOGY/School of Medicine/Department of Health Sciences
Source
First International Conference on Stepped Wedge Trial Design, York, UK