====== Cluster Sampling ======

Sometimes it is cheaper to 'cluster' the sample in some way e.g. by selecting respondents from certain areas only, or certain time-periods only. (Nearly all samples are in some sense 'clustered' in time -- although this is rarely taken into account in the analysis.)

===== Animation =====

<ani cs http://s288.photobucket.com/albums/ll181/xieyihui/cluster_sampling/ png 30 350 1|Demonstration of the cluster sampling>
Once a cluster is sampled, all its elements will be chosen.
</ani>

===== R code =====

<code r>
oopt = ani.options(ani.height = 350, ani.width = 500, outdir = getwd(), nmax = 30,
    interval = 1, title = "Demonstration of the cluster sampling",
    description = "Once a cluster is sampled, all its elements will be
    chosen.")
ani.start()
par(mar = rep(1, 4))
sample.cluster(col = c("bisque", "white"))
ani.stop()
ani.options(oopt)
</code>