"Ideally, a survey should gather data on every single person in the target population. For example, a survey about learning outcomes at a small school could track the test scores of every student. Collecting data on everyone in the target population is the best case scenario, since it ensures that everybody who matters to the survey is represented accurately. However, this is only possible if the population is small enough and the researchers have sufficient resources to reach out to everyone. This often is not the case, so researchers have to identify a subset of the population to survey. How you choose this subset of the target population is crucial to the quality of your data. The group must be carefully identified and representative of the larger population, else your data will not be useful for drawing inferences. If done right, survey sampling can save time and money while allowing you to draw interferences about a large group of people." (Introduction)