help for powerclus

Sample Size for Studies with Cluster Sampling

powerclus [
obsclus(#)
numclus(#)
rho(#)
]

Description

powerclus calculates sample size and number of clusters for cluster sampled
studies, correcting for any intraclass correlation. powerclus uses the
estimates from the "power" command, which must precede it.(Note: this is an
update from the program "samppclus" which worked with "sampsi")

Options

Note: must choose either "obsclus" or "numclus", but not both

obsclus(#) -- number of observations in each cluster; will return corrected
sample sizes and minimum number of clusters needed

Example: obsclus(10) -- 10 patients per physician; powerclus
determines number of physicians needed

numclus(#) -- maximum number of clusters; will return corrected sample
sizes and number of observations in each cluster. Number of obs.
per cluster are rounded up so that the number of clusters will
never exceed "numclus", but may be fewer. If too few clusters are
requested for the other given parameters, "powerclus" will prompt
for the minimum number of clusters possible.

Example: numclus(40) -- 40 physicians chosen; sampclus determines
number of patients per physician

rho(#) -- intraclass correlation coefficient (default=0)

Examples

. power twomeans 200 185, alpha(.01) power(.8) sd(30)

. powerclus, obsclus(10) rho(.2)

. powerclus, obsclus(10) rho(.1)

Corrects sample size and computes number of clusters from a t-test; adjusts
this sample size calculation for 10 observations per cluster and an
intraclass correlation of 0.2; repeats for an intraclass correlation of 0.1.

. power twoprop .20 .30, power(.8) nratio(2)

. powerclus, numclus(50) rho(.05)

Calculates sample size and number of observations per cluster using the
difference of two proportions with twice as many in the second group; assumes
the number of clusters is 50, and an intraclass correlation of 0.05.

Also see

Manual: [R] power

Author

J.Garrett, Professor, School of Medicine, University of North Carolina,
Chapel Hill, NC.
Email: joanne_garrett@med.unc.edu