sampling is conducted in stages; for a two-stage sample, the individuals are grouped
according to some characteristic—groups are first randomly selected and then individuals are randomly selected
from only those selected groups. (In a stratified sample, individuals are randomly selected from every group.)
For example, states could be randomly selected; then school districts within selected states, followed by schools
within selected school districts within selected states and finally students would be randomly selected from the
selected schools from the selected school districts from selected states. That would be a four-stage sample.