A sampling unit is an element or an individual in the target population. A sample is a subset of the population that is selected for the study. Before you use the survey procedures, you should have a well-defined target population, sampling units, and an appropriate sample design. In order to select a sample according to your sample design, you need to have a list of sampling units in the population. This is called a sampling frame .
Stratification controls the distribution of the sample size in the strata.
It is widely used in practice to meet a variety of survey objectives. For example, with stratification you can ensure adequate sample sizes for subgroups of interest, including small subgroups, or you can use stratification to improve the precision of overall estimates. To improve precision, units within strata should be as homogeneous as possible for the characteristics of interest. For example, clusters may be schools, hospitals, or geographical areas, and sampling units may be students, patients, or citizens. Cluster sampling can provide efficiency in frame construction and other survey operations.