A population consists of an entire set of objects, observations, or scores that have something in common. For example, a population might be defined as all males between the ages of 15 and 18.

Some populations are only hypothetical. Consider an experimenter interested in the possible effectiveness of a new method of teaching reading. He or she might define a population as the reading achievement scores that would result if all six year olds in the US were taught with this new method. The population is hypothetical in the sense that there does not exist a group of students who have been taught using the new method; the population consists of the scores that would be obtained if they were taught with this method.

The distribution of a population can be described by several parameters such as the mean and standard deviation. Estimates of these parameters taken from a sample are called statistics.