*Population* refers to the target population, which is the group of units (individuals or elements) of interest for study. Often, the
primary objective is to estimate certain characteristics of this population, which are called *population values*. A *sampling unit* is an individual or element 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*. PROC SURVEYSELECT uses probability-based selection methods to select a sample from a sampling frame.