then by gender. Once the groups are stratified by gender, random sam- ples are selected from each of the six subgroups.
Cluster Sampling When it is impossible
impractical to sample indi-
vidual elements from the population as a whole, usually when there is no exhaustive list of all the elements, cluster sampling is used. Cluster sampling involves the random selection of naturally occurring groups or
and then the selection of individual elements from the chosen groups or areas. Examples of naturally occurring groups would be uni- versities, schools, school divisions, classrooms, city blocks, and house-
holds. For example, if there is a need
survey a state for the television
viewing habits of middle school students, it would be cumbersome and difficult to select children at random from the state population of all
A clustering procedure could be employed by first listing all the school divisions in the state and then randomly selecting 30 school divisions from the list. One middle school could then be se- lected from each division, and students selected randomly from each school. This is a multistage clustering procedure. Although cluster pling saves time and money, the results are less accurate than other ran- dom sampling techniques.