Another step in planning the design is to identify the target population and select the sample if a census is not desired. The researcher must determine how many people to interview and who they will be; what and how many events to observe; how many records to inspect and which ones. Once the population of interest is determined, the researcher has to decide whether data will be collected from all study units or from some of the units in the population.
A sample is a part or a portion of the target population carefully selected to represent that population. When researchers undertake sampling studies, they are interested in estimating one or more population values and or testing one or more statistical hypothesis. The sampling process must give every person within a target population a known nonzero chance of selection if probability sampling is used. If there is no feasible alternative, a non profitability approach may be used.