The subjects we are measuring are called the observational units. We collect our data from the observational units. The measurement we take from each observational unit is called the variable, because we expect the results to vary from observational unit to observational unit. It is often helpful to think of the variable as contained in the questions posed to the observational units. The entire collection of observational units of interest is called the population, and the collection of observational units we actually gather data from--the (carefully chosen) subset of the population--is called the sample. It is often not feasible to measure every observational unit in the population, so we hope to use the sample to address our statistical investigation question, through inference