Design Effect for Cluster Sampling and Cluster Randomised Trials
design_effect.Rd
The design effect is the ratio of the total number of subjects required using cluster randomisation to the number required using individual randomisation.
Details
The design effect can be presented neatly in terms of the intracluster correlation and the number in a single cluster: -
$$D = 1 + (m − 1)r_{I}$$
If there is only one observation per cluster, m = 1
, the design effect is 1.0 and the two designs are the same.
Otherwise, the larger the intracluster correlation — that is, the more important the variation between clusters is — the
bigger the design effect and the more subjects we will need to get the same power as a simply randomised study. Even a
small intracluster correlation will have an impact if the cluster size is large.
References
Kerry, S.M. & Bland, J.M., 1998. Sample size in cluster randomisation. Brit Med J 316: 5490. doi:10.1136/bmj.316.7130.549 .
Kerry, S.M. & Bland, J.M., 1998. The intracluster correlation coefficient in cluster randomisation. Brit Med J 316: 1455-1460. doi:10.1136/bmj.316.7142.1455 .
See also
Other sample-size:
sample_size()