Why Sample?

Early in Chapter 2 we noted that we analyze data because we want to understand something about variation within a population or a process, and that more often than not we work with just a subset of the target population or process. Such a subset is known as a sample, and in this chapter we focus on the variability across possible samples from a single population.

In most cases it is impossible or impractical to gather data from every single individual within a population. In Chapter 2 we considered several sampling methods. At that time, we also presented the following sequence that describes the situation in many statistical studies: