6
Chapter 2
Sampling Wizard: Design Variables
Figure 2-2
Sampling Wizard, Design Variables step
This step allows you to select stratification and clustering variables and to define input sample
weights. You can also specify a label for the stage.
StratifyBy. The cross-classification of stratification variables definesdistinct subpopulations, or
strata. Separate samples are obtained for each stratum. To improve the precision of your estimates,
units withinstrata should be as homogeneous as possible for the characteristics of interest.
Clusters. Cluster variablesdefine groups of observational units, or clusters. Clustersare useful
when directly sampling observational units from the population is expensive or impossible;
instead, you can sample clustersfrom the population and then sample observational units from
the selected clusters. However, the use of clusters can introduce correlations among sampling
units, resulting in a loss ofprecision. Tominimize this effect, units within clusters should be as
heterogeneous as possible forthe characteristicsof interest. Youmust define at least one cluster
variable in order to plan a multistage design. Clusters are also necessary in the use of several
different sampling methods. For more information, see the topic Sampling Wizard: Sampling
Method on p. 8.