Population

A population includes every member of the group in question, not just the ones you happen to observe. For example, if you want the average height of all students at a school, the population is all of those students, and its true mean height is a fixed number you usually cannot measure directly. A numerical fact about a population, such as the population mean (the Greek letter mu, written $\mu$), is called a parameter. You typically estimate it from a smaller sample.