Population vs Parameter

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.

A parameter is a constant for a given population, even though you usually do not know its value. For example, the true proportion of all adults who own a car is a population proportion, written $p$ (the letter p). Parameters are typically labeled with Greek letters, such as the population mean $\mu$ (mu) and population standard deviation $\sigma$ (sigma). You estimate a parameter using a statistic computed from a sample.